testsuite: Ignore stderr in PartialDownsweep

As described in #17449, PartialDownsweep is currently fragile due to its
dependence on the error messages produced by the C preprocessor. To eliminate
this dependence we simply ignore stderr output, instead relying on the fact
that the test will exit with a non-zero exit code on failure.

Fixes #17449.

testsuite/tests/ghc-api/downsweep/PartialDownsweep.darwin.stderr

-== Parse error in export list
-== Parse error in export list with bypass module
-== Parse error in import list
-== CPP preprocessor error
-
-B.hs:2:2:  #elif without #if
-     #elif <- cpp error here
- ^
-1 error generated.
-== CPP preprocessor error with bypass
-
-B.hs:2:2:  #elif without #if
-     #elif <- cpp error here
- ^
-1 error generated.
-== Import error

testsuite/tests/ghc-api/downsweep/PartialDownsweep.stderr

-== Parse error in export list
-== Parse error in export list with bypass module
-== Parse error in import list
-== CPP preprocessor error
-
-B.hs:2:0: error:
-     error: #elif without #if
-     #elif <- cpp error here
-     
-== CPP preprocessor error with bypass
-
-B.hs:2:0: error:
-     error: #elif without #if
-     #elif <- cpp error here
-     
-== Import error

testsuite/tests/ghc-api/downsweep/all.T

 test('PartialDownsweep',
      [ extra_run_opts('"' + config.libdir + '"')
-     , when(opsys('darwin'),
-             use_specs({'stderr' : 'PartialDownsweep.darwin.stderr'})
-           )
+     , ignore_stderr
      ],
      compile_and_run,
      ['-package ghc'])