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

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

Fix servo#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.

resources/ahem.css

@@ -0,0 +1,4 @@
+@font-face {
+    font-family: Ahem;
+    src: url(ahem/AHEM____.TTF);
+}

tests/wpt/README.md

@@ -142,7 +142,7 @@ Reftest results can be analyzed from a raw log file. To generate this run
 with the `--log-raw` option e.g.
 
     ./mach test-css --log-raw css.log
-    
+
 This file can then be fed into the
 [reftest analyzer](http://hoppipolla.co.uk/410/reftest-analyser-structured.xhtml)
 which will show all failing tests (not just those with unexpected results).

tests/wpt/harness/wptrunner/browsers/servo.py

@@ -26,7 +26,8 @@ def check_args(**kwargs):
 
 def browser_kwargs(**kwargs):
     return {"binary": kwargs["binary"],
-            "debug_info": kwargs["debug_info"]}
+            "debug_info": kwargs["debug_info"],
+            "user_stylesheets": kwargs.get("user_stylesheets")}
 
 
 def executor_kwargs(test_type, server_config, cache_manager, run_info_data,
@@ -44,11 +45,13 @@ def env_options():
 
 
 class ServoBrowser(NullBrowser):
-    def __init__(self, logger, binary, debug_info=None):
+    def __init__(self, logger, binary, debug_info=None, user_stylesheets=None):
         NullBrowser.__init__(self, logger)
         self.binary = binary
         self.debug_info = debug_info
+        self.user_stylesheets = user_stylesheets or []
 
     def executor_browser(self):
         return ExecutorBrowser, {"binary": self.binary,
-                                 "debug_info": self.debug_info}
+                                 "debug_info": self.debug_info,
+                                 "user_stylesheets": self.user_stylesheets}

tests/wpt/harness/wptrunner/executors/executorservo.py

@@ -62,9 +62,10 @@ def do_test(self, test):
         self.result_data = None
         self.result_flag = threading.Event()
 
-        debug_args, command = browser_command(self.binary,
-            ["--cpu", "--hard-fail", "-u", "Servo/wptrunner", "-z", self.test_url(test)],
-            self.debug_info)
+        args = ["--cpu", "--hard-fail", "-u", "Servo/wptrunner", "-z", self.test_url(test)]
+        for stylesheet in self.browser.user_stylesheets:
+            args += ["--user-stylesheet", stylesheet]
+        debug_args, command = browser_command(self.binary, args, self.debug_info)
 
         self.command = command
 
@@ -192,6 +193,8 @@ def screenshot(self, test):
             self.command = [self.binary, "--cpu", "--hard-fail", "--exit",
                             "-u", "Servo/wptrunner", "-Z", "disable-text-aa",
                             "--output=%s" % output_path, full_url]
+            for stylesheet in self.browser.user_stylesheets:
+                self.command += ["--user-stylesheet", stylesheet]
 
             env = os.environ.copy()
             env["HOST_FILE"] = self.hosts_path

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)")