cmd/go: avoid stamping VCS metadata in test binaries
Invoking a VCS tool requires that the VCS tool be installed, and also
adds latency to build commands. Unfortunately, we had been mistakenly
loading VCS metadata for tests of "main" packages.
Users almost never care about versioning for test binaries, because
'go test' runs the test in the source tree and test binaries are only
rarely used outside of 'go test'. So the user already knows exactly
which version the test is built against, because the source code is
right there — it's not worth the overhead to stamp.