Tests: Test on jQuery 3.1.1.slim in non-BrowserStack browser tests

jQuery <3.2.0 doesn't include the deprecated module in the slim build so it
makes sense to test on one of these versions in slim mode even on PRs.

Author: mgol

jtr-local.yml

@@ -14,6 +14,7 @@ flags:
     - 3.3.1
     - 3.2.1
     - 3.1.1
+    - 3.1.1.slim
     - 3.0.0
 
 retries: 1