Only run tests on the "premium" build

It is the same as the Fdroid build, only with Dropbox support added.
Testing both builds takes forever, and the only difference is Dropbox
tests are not run when testing the Fdroid build.

.github/workflows/test.yaml

@@ -74,4 +74,5 @@ jobs:
           disable-animations: true
           disable-spellchecker: true
           profile: Nexus 6
-          script: ./gradlew connectedCheck
+          # Run tests against the build with Dropbox included
+          script: ./gradlew connectedPremiumDebugAndroidTest --stacktrace