Python: Make test/Filter query more robust

Since if you had tornado installed, we would follow imports and have
results from those files as well :|

python/ql/test/library-tests/filters/tests/Filter.ql

@@ -2,4 +2,5 @@ import python
 import semmle.python.filters.Tests
 
 from TestScope t
+where exists(t.getLocation().getFile().getRelativePath())
 select t