Bug 1975349 - Use "sync" variable to disable tests on builds without Sync. a=RyanVM DONTBUILD

Sync is going to be disabled on Thunderbird again for a time, so these tests would need disabling.
Instead, I'm changing the condition to use the sync variable, so they are automatically enabled on
builds with Sync and disabled on builds without.

Original Revision: https://phabricator.services.mozilla.com/D255900

Differential Revision: https://phabricator.services.mozilla.com/D260073

services/fxaccounts/tests/xpcshell/xpcshell.toml

@@ -1,7 +1,7 @@
 [DEFAULT]
 head = "head.js ../../../common/tests/unit/head_helpers.js ../../../common/tests/unit/head_http.js"
 firefox-appdir = "browser"
-skip-if = ["appname == 'thunderbird' && !nightly_build"]
+skip-if = ["!sync"]
 support-files = [
   "!/services/common/tests/unit/head_helpers.js",
   "!/services/common/tests/unit/head_http.js",

toolkit/components/extensions/test/xpcshell/xpcshell.toml

@@ -48,7 +48,10 @@ run-if = ["os == 'android'"] # Android has no remote extensions, Bug 1535365
 ["test_OriginControls.js"]
 
 ["test_StorageSyncService.js"]
-skip-if = ["os == 'android' && processor == 'x86_64'"]
+skip-if = [
+  "!sync",
+  "os == 'android' && processor == 'x86_64'",
+]
 
 ["test_WebExtensionPolicy.js"]