test: fix parametrisation support in subprocess marker [backport 1.14] (#6087)

Backport 5d174c8 from #6085 to 1.14.

A bug in the handling of the parametrize argument to the subprocess
marker prevented it from generating the correct test matrix. This fix
ensures that the correct matrix dictionary is generated so that all the
intended test cases are actually run.

Author: github-actions[bot]

tests/conftest.py

@@ -147,7 +147,7 @@ def unwind_params(params):
         yield None
         return
 
-    for _ in product(*(((k, v) for v in vs) for k, vs in params.items())):
+    for _ in product(*([(k, v) for v in vs] for k, vs in params.items())):
         yield dict(_)