Hide error from expected failing parallel GC test

Summary:
This test spawns a subprocess and expects it to fail.  However when it fails it
prints the exception to the stderr of the parent process, which causes confusion
when we're debugging other failures.  Hide it by default.

Reviewed By: yoney

Differential Revision: D83001409

fbshipit-source-id: 5a5cc4a2c4dcd6792864abcd0ac9118f3a8787fd

Author: alexmalyshev

cinderx/PythonLib/test_cinderx/test_enabling_parallel_gc.py

@@ -92,6 +92,7 @@ def g():
                 args,
                 cwd=tmp,
                 stdout=subprocess.PIPE,
+                stderr=subprocess.PIPE,
                 encoding=ENCODING,
                 env={
                     "PYTHONPATH": CINDERX_PATH,