Add TestingBuildParams.RelativeToTop()

The methods returns a copy of TestingBuildParams with every usage of a
path that is relative to the temporary test build directory with a path
relative to a notional top. Similar to how PathRelativeToTop does.

Bug: 182885307
Test: m nothing
Change-Id: I6ec20fc52ed76748138f0d48b4df80f765dfcfdc
diff --git a/android/namespace_test.go b/android/namespace_test.go
index 604b356..08e221a 100644
--- a/android/namespace_test.go
+++ b/android/namespace_test.go
@@ -697,7 +697,7 @@
 		testModule, ok := candidate.(*testModule)
 		if ok {
 			if testModule.properties.Id == id {
-				module = newTestingModule(testModule)
+				module = newTestingModule(ctx.config, testModule)
 			}
 		}
 	}