Bug 1667998 - [ci] Add cli flag in desktop_unittest.py. Ensure M-a11y-checks runs on Linux x64 opt build. r=jmaher

Differential Revision: https://phabricator.services.mozilla.com/D102042

taskcluster/ci/test/mochitest.yml

@@ -114,7 +114,7 @@ mochitest-browser-chrome:
     loopback-video: true
     variants:
         by-test-platform:
-            linux.*64-.*/opt: ['fission', 'a11y-checks']
+            linux.*64(-shippable)?/opt: ['fission', 'a11y-checks']
             default: ['fission']
     fission-run-on-projects:
         by-test-platform:
@@ -213,7 +213,7 @@ mochitest-devtools-chrome:
     loopback-video: true
     variants:
         by-test-platform:
-            linux.*64-.*/opt: ['fission', 'a11y-checks']
+            linux.*64(-shippable)?/opt: ['fission', 'a11y-checks']
             default: ['fission']
     fission-run-on-projects:
         by-test-platform:

taskcluster/taskgraph/transforms/tests.py

@@ -178,6 +178,15 @@ TEST_VARIANTS = {
     "a11y-checks": {
         "description": "{description} with accessibility checks enabled",
         "suffix": "a11y-checks",
+        "replace": {
+            "run-on-projects": {
+                "by-test-platform": {
+                    "linux.*64(-shippable)?/opt": ["trunk"],
+                    "default": [],
+                },
+            },
+            "tier": 2,
+        },
         "merge": {
             "mozharness": {
                 "extra-options": [
@@ -185,7 +194,6 @@ TEST_VARIANTS = {
                 ],
             },
         },
-        "tier": 2,
     },
     "geckoview-e10s-single": {
         "description": "{description} with single-process e10s",

testing/mozharness/scripts/desktop_unittest.py

@@ -260,6 +260,15 @@ class DesktopUnittest(TestingMixin, MercurialScript, MozbaseMixin, CodeCoverageM
                     "help": "Run tests in a cross origin iframe.",
                 },
             ],
+            [
+                ["--enable-a11y-checks"],
+                {
+                    "action": "store_true",
+                    "default": False,
+                    "dest": "a11y_checks",
+                    "help": "Run tests with accessibility checks disabled.",
+                },
+            ],
         ]
         + copy.deepcopy(testing_config_options)
         + copy.deepcopy(code_coverage_config_options)
@@ -585,6 +594,9 @@ class DesktopUnittest(TestingMixin, MercurialScript, MozbaseMixin, CodeCoverageM
             if c["extra_prefs"]:
                 base_cmd.extend(["--setpref={}".format(p) for p in c["extra_prefs"]])
 
+            if c["a11y_checks"]:
+                base_cmd.append("--enable-a11y-checks")
+
             # set pluginsPath
             abs_res_plugins_dir = os.path.join(abs_res_dir, "plugins")
             str_format_values["test_plugin_path"] = abs_res_plugins_dir