Previously if the only names defined by _test packages were examples,
the gotest script would emit an incorrect _testmain.go file.
I worked around that by marking the example_test.go files +build ignored.
This CL changes the gotest script to handle this case correctly,
and removes the now-unnecessary build tags.
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/214039
From-SVN: r280085
fcee603081