Bug 1455824 - Disable TEST 18 of test_font_loading_api.html on Android for fragile timeout behavior r=emilio

TEST 18 of layout/style/test/test_font_loading_api.html is fragile on Android in the sense
that adding a new mochitest in an irrelevant directory (e.g. layout/svg/test)
might cause a failure rate close to 50%. See Comment 34 in Bug 1455824.
Increasing SimpleTest.requestLongerTimeout() doesn't make much difference, see
https://treeherder.mozilla.org/#/jobs?repo=try&revision=c954cc8621372252a500fe1db04fb06edd636d9c.

Comment 43 and 44 from Testing team suggested the drastical failure increasing
is probably only related to different task grouping when a new testcase is added
elsewhere.

We should probably disable it now on Android platform, because it might cause
false-negative and confusion when someone lands an unrelated mochitest in
an irrelevant directory.

Differential Revision: https://phabricator.services.mozilla.com/D25718

--HG--
extra : moz-landing-system : lando

layout/style/test/test_font_loading_api.html

@@ -409,6 +409,13 @@ function runTest() {
     // (TEST 18) Test passing valid url() source strings to the FontFace
     // constructor.
     var p = Promise.resolve();
+
+    // The sub-test is very fragile on Android platform, see Bug 1455824,
+    // especially Comment 34.
+    if (navigator.appVersion.includes("Android")) {
+      return p;
+    }
+
     sourceWindows.forEach(function({ win, what }) {
       p = p.then(function() {
         var srcTests = Promise.resolve();