run.bash: set GOPATH to $GOROOT/nil before running tests
run.bash: set GOPATH to $GOROOT/nil before running tests

Set $GOPATH to a semantically valid, non-empty string that cannot
conflict with $GOROOT to avoid false test failures that occur when
$GOROOT resides under $GOPATH.  Unsetting GOPATH is no longer viable
as Go now defines a default $GOPATH that may conflict with $GOROOT.

Fixes #19237

Change-Id: I376a2ad3b18e9c4098211b988dde7e76bc4725d2
Reviewed-on: https://go-review.googlesource.com/37396
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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