Reenable problematic test

Author: tamasvajk

csharp/ql/integration-tests/all-platforms/diag_missing_project_files/test.py

@@ -1,8 +1,2 @@
-import pytest
-import runs_on
-
-
-# Skipping the test on macos-15, as we're running into trouble.
-@pytest.mark.only_if(not runs_on.macos_15)
 def test(codeql, csharp):
     codeql.database.create(_assert_failure=True)