cmd/go: exit codes in packages with tests but no source files

[broady]

go get makes it difficult to have a directory with tests but no source files.

For example, golang.org/x/tour/content has content_test.go but no other Go files.

$ go get -t golang.org/x/tour/...; echo $?
go build golang.org/x/tour/content: no buildable Go source files in /usr/local/google/home/cbro/src/golang.org/x/tour/content
1

go version devel +edb19aa Fri Mar 25 18:35:15 2016 +0000 linux/amd64

[rsc]

This would be nice to fix if it is trivial. It may not be.

[gopherbot]

CL https://golang.org/cl/23314 mentions this issue.