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

libgo/go/database/sql/example_cli_test.go

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build ignore
-
 package sql_test
 
 import (