tests: Use Ahem font provided by WPT harness. (#37054)

#37021 exposed that our tests are trying to load a local file path for
Ahem.ttf, but this stops working when we implement the specified CORS
protections for our font requests. Luckily, the WPT suite already
provides a same-origin version of the same font, so we can just update
our CSS to use that font face instead.

Testing: Existing WPT coverage.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

components/net/tests/fetch.rs

@@ -225,7 +225,7 @@ fn test_fetch_blob() {
 
 #[test]
 fn test_file() {
-    let path = Path::new("../../resources/ahem.css")
+    let path = Path::new("../../components/net/tests/test.css")
         .canonicalize()
         .unwrap();
     let url = ServoUrl::from_file_path(path.clone()).unwrap();

components/net/tests/test.css

@@ -0,0 +1,3 @@
+html {
+    color: red;
+}