Bug 1750638 - Update the mozversioncontrol test CI job to run also run on Mac and Windows r=firefox-build-system-reviewers,mhentges

The platform(s) were not specified, and it seems when that is the case that
it will only run on linux by default. The platforms are now explicitly
set to include Windows, Linux, and Mac.

Differential Revision: https://phabricator.services.mozilla.com/D136214

taskcluster/ci/source-test/python.yml

@@ -249,6 +249,10 @@ mozterm:
 
 mozversioncontrol:
     description: python/mozversioncontrol unit tests
+    platform:
+        - linux1804-64/opt
+        - macosx1015-64/opt
+        - windows10-64/opt
     python-version: [3]
     treeherder:
         symbol: vcs