buildserver: test builds against fdroid and adaway

The other apps are too flaky on gpjenkins right now, and that's our
only box for running full buildserver tests.  Once we get the
buildserver tests running on jenkins.debian.net, then we can add a
bunch more apps to the test script.  gpjenkins is an extra locked down
box, so that's why the builds are flaky: gradle and maven downloads
regularly fail because they are blocked.