minimize webpack-builder cache dependencies

refs DE-1379

Change-Id: I99987df025139cc9b9684d4d2a8073c1c7c94360
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/302329
Reviewed-by: Andrea Cirulli <andrea.cirulli@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
QA-Review: Aaron Ogata <aogata@instructure.com>
Product-Review: Aaron Ogata <aogata@instructure.com>
Build-Review: Aaron Ogata <aogata@instructure.com>

Dockerfile.jenkins-cache

@@ -35,6 +35,9 @@ RUN --mount=target=/tmp/src \
     | tee /tmp/dst/yarn-runner.tar | md5sum > /tmp/dst/yarn-runner.tar.md5 && \
   \
   tar --sort=name --mtime='1970-01-01' --owner=0 --group=0 --numeric-owner --mode='a+rwX' -cf - \
+    --exclude='**/__mocks__' \
+    --exclude='**/__tests__' \
+    --exclude='**/*.test.js' \
     ui/engine \
     packages \
     \