Test the case where the package has the `_test` suffix.

Prior to patch:

    $ go generate
    $ go test
    # github.com/golang/mock/mockgen/internal/tests/mock_in_test_package_test [github.com/golang/mock/mockgen/internal/tests/mock_in_test_package.test]
    ./mock_test.go:36:44: undefined: User
    ./mock_test.go:38:21: undefined: User
    FAIL	github.com/golang/mock/mockgen/internal/tests/mock_in_test_package [build failed]

With this patch applied:

    $ go generate
    $ go test
    ok  	github.com/golang/mock/mockgen/internal/tests/mock_in_test_package	0.031s
