Revert "populate-app.sh: skip tests when building the app"

This reverts commit 71e0d8cc3b213a6a7e30fc9ee976d66e2381e84c.

Johannes requested that tests not be skipped by default, because it is
sometimes useful for them to run in certain contexts (e.g., before
uploading). But populate-app.sh accepts arguments which are passed to
the mvn executable, so you can run:

    bin/populate-app.sh -DskipTests

If you really don't want to run them.