Fix failing test T12504

Summary:
Test T12504 does it's checking in the Makefile using grep but still specified an stdout.
the stdout has the old output and would always fail.

Since the stdout isn't needed, let's not check it.

Test Plan: make test TEST=T12504

Reviewers: bgamari, austin, erikd, rcook

Reviewed By: rcook

Subscribers: thomie, #ghc_windows_task_force

Differential Revision: https://phabricator.haskell.org/D2537

GHC Trac Issues: #12504

testsuite/tests/hsc2hs/T12504.stdout

-{-# LINE 1 "T12504/path/to/T12504.hsc" #-}
-{-# LINE 2 "T12504/path/to/T12504.hsc" #-}

testsuite/tests/hsc2hs/all.T

@@ -38,6 +38,7 @@ test('T10272',
 
 test('T12504',
      [extra_clean(['T12504/path/to/T12504.hs']),
-      extra_files(['T12504'])],
+      extra_files(['T12504']),
+      ignore_stdout],
      run_command,
      ['$MAKE -s --no-print-directory T12504'])