os/exec: re-enable TestExtraFiles for plan9
os/exec: re-enable TestExtraFiles for plan9

This test should now succeed after CL 22610 which fixes issue #7118

Change-Id: Ie785a84d77b27c832a1ddd81699bf25dab24b97d
Reviewed-on: https://go-review.googlesource.com/22640
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: David du Colombier <ec8ae1e6620d4bab5e0e7aa2a6ae41aad63e3708@gmail.com>
Run-TryBot: David du Colombier <ec8ae1e6620d4bab5e0e7aa2a6ae41aad63e3708@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/go,cloudflare/go,vsekhar/elastic-go,Cofyc/go,golang/go,linux-on-ibm-z/go,4ad/go,CAFxX/go,ganboing/go-esx,polyverse-security/go,CAFxX/go,vsekhar/elastic-go,ganboing/go-esx,derekmarcotte/go,vsekhar/elastic-go,ganboing/go-esx,derekmarcotte/go,josharian/go,nawawi/go,Cofyc/go,akutz/go,polyverse-security/go,ImJasonH/go,golang/go,polyverse-security/go,ImJasonH/go,Cofyc/go,golang/go,cloudflare/go,josharian/go,ImJasonH/go,4ad/go,ganboing/go-esx,josharian/go,momchil-velikov/go,theif519/go,josharian/go,CAFxX/go,golang/go,momchil-velikov/go,cloudflare/go,polyverse-security/go,CAFxX/go,josharian/go,vsekhar/elastic-go,linux-on-ibm-z/go,derekmarcotte/go,CAFxX/go,goodlang/good,4ad/go,codestation/go,momchil-velikov/go,akutz/go,goodlang/good,josharian/go,christopher-henderson/Go,theif519/go,vsekhar/elastic-go,cloudflare/go,nawawi/go,linux-on-ibm-z/go,ganboing/go-esx,nawawi/go,derekmarcotte/go,cloudflare/go,ganboing/go-esx,theif519/go,linux-on-ibm-z/go,golang/go,goodlang/good,theif519/go,cloudflare/go,suncycheng/go,goodlang/good,ganboing/go-esx,akutz/go,theif519/go,suncycheng/go,polyverse-security/go,nawawi/go,momchil-velikov/go,nawawi/go,akutz/go,vsekhar/elastic-go,cloudflare/go,ganboing/go-esx,Cofyc/go,akutz/go,vsekhar/elastic-go,codestation/go,4ad/go,akutz/go,ImJasonH/go,cloudflare/go,ImJasonH/go,ImJasonH/go,vsekhar/elastic-go,codestation/go,theif519/go,theif519/go,akutz/go,momchil-velikov/go,golang/go,momchil-velikov/go,linux-on-ibm-z/go,linux-on-ibm-z/go,codestation/go,4ad/go,momchil-velikov/go,4ad/go,golang/go,polyverse-security/go,suncycheng/go,codestation/go,ImJasonH/go,Cofyc/go,ImJasonH/go,codestation/go,suncycheng/go,suncycheng/go,nawawi/go,codestation/go,suncycheng/go,theif519/go,linux-on-ibm-z/go,goodlang/good,polyverse-security/go,suncycheng/go,codestation/go,linux-on-ibm-z/go,derekmarcotte/go,suncycheng/go,CAFxX/go,derekmarcotte/go,nawawi/go,josharian/go,Cofyc/go,nawawi/go,derekmarcotte/go,goodlang/good,Cofyc/go,polyverse-security/go,josharian/go,CAFxX/go,goodlang/good,Cofyc/go,4ad/go,derekmarcotte/go,4ad/go,CAFxX/go,akutz/go,goodlang/good,momchil-velikov/go