mk: Add check-secondary target for tests with no x-platform risk.
mk: Add check-secondary target for tests with no x-platform risk.

We'll use this to run a subset of the test suite onto a dedicated
bot.

This puts the grammar tests and the pretty-printer tests under
check-secondary. It leanves the pretty tests under plain `check`
for now, until the new bot is added to take over.

Because check-secondary is not run as part of `make check` there
will be a set of tests that most users never run and are only
checked by bors. I think this will be ok because grammar tests
should rarely regress, and the people regressing such tests
should have the fortitude to deal with it.

jashank/rust,mahkoh/rust,rprichard/rust,philyoon/rust,carols10cents/rust,krzysz00/rust,kwantam/rust,untitaker/rust,GBGamer/rust,vhbit/rust,barosl/rust,ejjeong/rust,aneeshusa/rust,P1start/rust,dwillmer/rust,aneeshusa/rust,ejjeong/rust,richo/rust,mihneadb/rust,andars/rust,kwantam/rust,graydon/rust,mvdnes/rust,AerialX/rust-rt-minimal,pelmers/rust,KokaKiwi/rust,rprichard/rust,ktossell/rust,P1start/rust,ejjeong/rust,aidancully/rust,dinfuehr/rust,graydon/rust,aneeshusa/rust,gifnksm/rust,seanrivera/rust,victorvde/rust,nham/rust,graydon/rust,barosl/rust,KokaKiwi/rust,cllns/rust,cllns/rust,l0kod/rust,miniupnp/rust,l0kod/rust,mvdnes/rust,reem/rust,rprichard/rust,ejjeong/rust,GBGamer/rust,mihneadb/rust,jashank/rust,carols10cents/rust,cllns/rust,P1start/rust,XMPPwocky/rust,zaeleus/rust,zubron/rust,AerialX/rust,GBGamer/rust,hauleth/rust,rprichard/rust,KokaKiwi/rust,nwin/rust,XMPPwocky/rust,emk/rust,reem/rust,aidancully/rust,TheNeikos/rust,quornian/rust,TheNeikos/rust,jroesch/rust,GBGamer/rust,l0kod/rust,krzysz00/rust,jroesch/rust,philyoon/rust,jroesch/rust,nwin/rust,defuz/rust,jbclements/rust,0x73/rust,mvdnes/rust,kimroen/rust,hauleth/rust,philyoon/rust,XMPPwocky/rust,gifnksm/rust,rohitjoshi/rust,erickt/rust,aepsil0n/rust,zachwick/rust,ebfull/rust,P1start/rust,jbclements/rust,quornian/rust,omasanori/rust,sae-bom/rust,rprichard/rust,rohitjoshi/rust,defuz/rust,robertg/rust,aidancully/rust,AerialX/rust,kimroen/rust,erickt/rust,philyoon/rust,ktossell/rust,mvdnes/rust,victorvde/rust,reem/rust,jashank/rust,miniupnp/rust,0x73/rust,zachwick/rust,quornian/rust,AerialX/rust-rt-minimal,bombless/rust,hauleth/rust,XMPPwocky/rust,nwin/rust,carols10cents/rust,jbclements/rust,vhbit/rust,robertg/rust,pshc/rust,carols10cents/rust,avdi/rust,pshc/rust,victorvde/rust,zaeleus/rust,nwin/rust,mihneadb/rust,KokaKiwi/rust,rohitjoshi/rust,kimroen/rust,cllns/rust,vhbit/rust,ebfull/rust,AerialX/rust-rt-minimal,miniupnp/rust,philyoon/rust,mihneadb/rust,nwin/rust,robertg/rust,robertg/rust,untitaker/rust,ruud-v-a/rust,mahkoh/rust,dinfuehr/rust,dwillmer/rust,philyoon/rust,emk/rust,sae-bom/rust,miniupnp/rust,robertg/rust,TheNeikos/rust,omasanori/rust,ktossell/rust,ruud-v-a/rust,bombless/rust,quornian/rust,gifnksm/rust,zubron/rust,kimroen/rust,gifnksm/rust,ktossell/rust,bombless/rust-docs-chinese,ruud-v-a/rust,0x73/rust,nham/rust,defuz/rust,AerialX/rust-rt-minimal,pshc/rust,rohitjoshi/rust,zubron/rust,reem/rust,GBGamer/rust,ebfull/rust,quornian/rust,untitaker/rust,vhbit/rust,avdi/rust,graydon/rust,jashank/rust,aidancully/rust,ktossell/rust,l0kod/rust,miniupnp/rust,gifnksm/rust,richo/rust,avdi/rust,l0kod/rust,nham/rust,P1start/rust,jbclements/rust,hauleth/rust,kimroen/rust,victorvde/rust,ejjeong/rust,zachwick/rust,aepsil0n/rust,erickt/rust,kwantam/rust,andars/rust,TheNeikos/rust,aepsil0n/rust,kimroen/rust,victorvde/rust,kimroen/rust,victorvde/rust,AerialX/rust-rt-minimal,carols10cents/rust,mahkoh/rust,untitaker/rust,avdi/rust,ktossell/rust,zubron/rust,dinfuehr/rust,vhbit/rust,pelmers/rust,dwillmer/rust,XMPPwocky/rust,richo/rust,rohitjoshi/rust,barosl/rust,GBGamer/rust,KokaKiwi/rust,ejjeong/rust,miniupnp/rust,dwillmer/rust,XMPPwocky/rust,vhbit/rust,AerialX/rust,pelmers/rust,ruud-v-a/rust,rprichard/rust,dwillmer/rust,omasanori/rust,jroesch/rust,seanrivera/rust,seanrivera/rust,aepsil0n/rust,zaeleus/rust,krzysz00/rust,pelmers/rust,TheNeikos/rust,zubron/rust,aepsil0n/rust,pshc/rust,erickt/rust,mdinger/rust,cllns/rust,seanrivera/rust,pelmers/rust,mahkoh/rust,gifnksm/rust,quornian/rust,aidancully/rust,zaeleus/rust,untitaker/rust,avdi/rust,emk/rust,bombless/rust,jroesch/rust,hauleth/rust,mvdnes/rust,aneeshusa/rust,aneeshusa/rust,jbclements/rust,nham/rust,mihneadb/rust,aneeshusa/rust,kwantam/rust,graydon/rust,avdi/rust,mvdnes/rust,hauleth/rust,0x73/rust,KokaKiwi/rust,krzysz00/rust,miniupnp/rust,bombless/rust,0x73/rust,carols10cents/rust,seanrivera/rust,jbclements/rust,zachwick/rust,dinfuehr/rust,nwin/rust,barosl/rust,GBGamer/rust,P1start/rust,jbclements/rust,emk/rust,barosl/rust,rohitjoshi/rust,defuz/rust,richo/rust,vhbit/rust,ebfull/rust,zubron/rust,zaeleus/rust,sae-bom/rust,dwillmer/rust,bombless/rust,GBGamer/rust,omasanori/rust,emk/rust,andars/rust,0x73/rust,graydon/rust,emk/rust,dwillmer/rust,richo/rust,mdinger/rust,pshc/rust,andars/rust,sae-bom/rust,quornian/rust,pshc/rust,vhbit/rust,omasanori/rust,barosl/rust,aepsil0n/rust,zachwick/rust,barosl/rust,kwantam/rust,AerialX/rust,pelmers/rust,erickt/rust,jroesch/rust,aidancully/rust,sae-bom/rust,mdinger/rust,ruud-v-a/rust,l0kod/rust,TheNeikos/rust,richo/rust,pshc/rust,nwin/rust,l0kod/rust,zubron/rust,defuz/rust,AerialX/rust,reem/rust,bombless/rust,AerialX/rust,pshc/rust,robertg/rust,ebfull/rust,reem/rust,mdinger/rust,AerialX/rust-rt-minimal,emk/rust,P1start/rust,ruud-v-a/rust,jroesch/rust,jroesch/rust,erickt/rust,zubron/rust,jashank/rust,l0kod/rust,ebfull/rust,nham/rust,0x73/rust,seanrivera/rust,krzysz00/rust,dwillmer/rust,untitaker/rust,dinfuehr/rust,nwin/rust,mdinger/rust,jashank/rust,omasanori/rust,mahkoh/rust,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