Make the Ahem font available to test-css and test-wpt tests.

Provide a user stylesheet with an `@font-face` rule for it.

Fix #6195.

Many previously-failing tests now pass, and a few previously-passing now fail.

Among the latter, `font-family-013.htm` and `fonts-013.htm` are testing
that the Ahem font is not used for characters it doesn’t have a glyph for.
They were passing because Ahem was not available at all,
and now fail because we don’t implement font fallback correctly.

The others also use Ahem, but I don’t understand yet what’s going on exactly.

tests/wpt/harness/wptrunner/wptcommandline.py

@@ -159,6 +159,11 @@ def create_parser(product_choices=None):
     b2g_group.add_argument("--b2g-no-backup", action="store_true", default=False,
                            help="Don't backup device before testrun with --product=b2g")
 
+    servo_group = parser.add_argument_group("Servo-specific")
+    servo_group.add_argument("--user-stylesheet",
+                             default=[], action="append", dest="user_stylesheets",
+                             help="Inject a user CSS stylesheet into every test.")
+
     parser.add_argument("test_list", nargs="*",
                         help="List of URLs for tests to run, or paths including tests to run. "
                              "(equivalent to --include)")