"go test go/types -run TestStdFixed" results may be cached by cmd/go even after changing files in $GOROOT/test. I noticed this by changing "ignored" to "package ignored" in test/fixedbugs/issue22877.go (in CL 80759), and repeated runs of go test still output "(cached)" pass results.
Per @rsc 's e-mail to golang-dev (re: Go 1.10 cmd/go: build cache, test cache, go install, go vet, test vet):
"The cache only applies to "go test package-list", not to "go test" without a package list. So if you're working in a directory and just keep running "go test", none of those are cached."
So it should work as expected from within the go/types directory.
That said, this is probably a general problem and perhaps there needs to be a way to tell the test system to not consider the caches, ideally by the tests themselves (somebody not familiar with a specific program, trying to identify an issue, may not be aware that caching interferes).
In my case, I was working in the $GOROOT/test directory, but discovered go/types tests that directory too. That's why I ran "go test go/types". I didn't expect the behavior to differ if I ran "cd ../src/go/types; go test".
I now see this is documented in "go doc cmd/go". I suppose it's time that I re-read it.