Run tests only when documentation has been built

Run tests only when documentation has been built

This was accomplished by splitting override_dh_auto_test into two,
i.e., an empty override_dh_auto_test-arch (hence disabled), and
an override_dh_auto_test-indep where the tests are actually run.

Also fix the check for DEB_BUILD_OPTIONS=nocheck.
Previously, the tests were run only when "nocheck" was defined.
See #760794 for previous discussions.