testing: add a method testing.CoverMode
testing: add a method testing.CoverMode

This makes it possible to avoid tests where coverage affects the test
results by skipping them (or otherwise adjusting them) when coverage
is enabled.

Update #17699

Change-Id: Ifcc36cfcd88ebd677890e82ba80ee3d696ed3d7c
Reviewed-on: https://go-review.googlesource.com/32483
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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