Bug 1608834 - [ci] Chunk mochitest-a11y and plain in the taskgraph, r=gbrown

Differential Revision: https://phabricator.services.mozilla.com/D58039

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

taskcluster/taskgraph/transforms/tests.py

@@ -1287,12 +1287,10 @@ CHUNK_SUITES_BLACKLIST = (
     'jittest',
     'jsreftest',
     'marionette',
-    'mochitest-a11y',
     'mochitest-browser-chrome',
     'mochitest-browser-chrome-screenshots',
     'mochitest-browser-chrome-thunderbird',
     'mochitest-devtools-webreplay',
-    'mochitest-plain',
     'mochitest-valgrind-plain',
     'mochitest-webgl1-core',
     'mochitest-webgl1-ext',