Avoid using string replacement on a bogus string to get the relative directory of this test.

https://github.com/flutter/engine/pull/46911 will fix the behavior of Platform.script so it actually has the file being executed, but that will not be the file the test author wrote, it will be `listener.dart.dill` generated by tool code and running in a temp directory for this particular test.

We may want to come up with a more robust way of providing the test for framework test users, but we should do so in a way that will not limit the ability to run multiple tests in a single process if that offers performance benefits.