Disable csharp test failing on macos-15

Author: oscarsj

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

@@ -1,2 +1,8 @@
+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)