Automatically call TestingBuildParams.RelativeToTop()

Fixes the few tests that break due to this and which cannot easily be
separated into their own changes.

Bug: 183650682
Test: m nothing
Change-Id: Ia2f31213a1f114a78e66a81d89279ecde9f4c465
diff --git a/cc/proto_test.go b/cc/proto_test.go
index f8bbd26..3d636fb 100644
--- a/cc/proto_test.go
+++ b/cc/proto_test.go
@@ -61,7 +61,7 @@
 			t.Errorf("expected %q in %q", w, cmd)
 		}
 
-		foobarPath := foobar.Module().(android.HostToolProvider).HostToolPath().String()
+		foobarPath := foobar.Module().(android.HostToolProvider).HostToolPath().RelativeToPath().String()
 
 		if w := "--plugin=protoc-gen-foobar=" + foobarPath; !strings.Contains(cmd, w) {
 			t.Errorf("expected %q in %q", w, cmd)