ci(unit tests): run Jest tests in single thread to prevent OOM exception

Since the circle container is only 4GB, running jest tests in parallel is problematic as the
container frequently runs out of memory. The memoryt can either be increased, or the tests can run
in a single thread

Author: AtofStryker

Diff for: .circleci/config.yml

@@ -50,7 +50,7 @@ jobs:
       - *restore_node_modules
       - run:
           name: yarn test:unit
-          command: yarn test:unit
+          command: yarn test:unit -w 1
 workflows:
   version: 2
   install-tests: