go/gcimporter15: fix build

This change prevents test code to slip into package gcimporter15 and
fixes a weird situation that the test passes but the package doesn't
exist.

For example, we can see the situation with Go 1.6 like the follwoing:

go build
./gcimporter_test16.go:20: undefined: MustHaveGoBuild
./gcimporter_test16.go:45: undefined: testPath

go test
PASS
ok  	golang.org/x/tools/go/gcimporter15	0.896s

Change-Id: I95550574ccd1b2273072c700c28a82c791c16c63
Reviewed-on: https://go-review.googlesource.com/18950
Reviewed-by: Brad Fitzpatrick <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>