libgo: compile examples in _test packages

    
    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