bugfix: make spack -d test test_changed_files work (#13229)

The `test_changed_files` in `test/cmd/flake8.py` was failing because it calls
`ArgumentParser.parse_args()` without arguments.  Normally that would just
parse `sys.argv` but it seems to fail because of something in either `spack test`
or `pytest`.  Call it with an empty array so that it doesn't try to touch`sys.argv`
at all.

- [x] allow `-d` spack option for `test_changed_files`

lib/spack/spack/test/cmd/flake8.py

@@ -45,7 +45,7 @@ def flake8_package():
 
 
 def test_changed_files(parser, flake8_package):
-    args = parser.parse_args()
+    args = parser.parse_args([])
 
     # changed_files returns file paths relative to the root
     # directory of Spack. Convert to absolute file paths.