go/types: automatically ignore $GOROOT/test files that contain build tags
These files were already ignored via a hard-coded list of excluded files.
Instead of trying to interpret the build tags for these (few) files,
recognize the tags automatically and continue to exclude them.
Fixes #10370.
Change-Id: If7a112ede02e3fa90afe303473d9ea51c5c6609d
Reviewed-on: https://go-review.googlesource.com/c/144457 Reviewed-by: Alan Donovan <adonovan@google.com>