testing: add clear panic for duplicate call to t.Parallel
testing: add clear panic for duplicate call to t.Parallel

Change-Id: I155633b58e1823344a26c3edf11f5626fae080ee
Reviewed-on: https://go-review.googlesource.com/18204
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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