This is a more flexible approach than testing for the complete SDK and
build-tools up front. This will only test for the commands that are
actually being run, so that if you only have `aapt` installed, you can do
`fdroid update` without errors, but other commands will still give
appropriate errors.
This also makes the build_tools item in config.py optional, it is only
needed if you want to force a specific version of the build-tools.
Just getting into the habit of adding tests to everything that I change...
Also, it should be useful to have an unsigned APK in the test collection,
since `fdroid update` should handle it gracefully and give a warning of
some kind.
