Make docker-tests less verbose and easier to add new tests

[skywhale]

• Reorganize tests into functions
• Replace userspace/kernel argument with --userspace option
• Add --verbose option (the test currently prints 3483 lines, which is a bit overwhelming)
• Take optional positional arguments to filter tests

[strohel]

Oh thanks! Looked good, though my eye is not trained to spot problems in bash.

But this is test infra, so not worth to delay this by asking more eyes on this, though if @bschwind (or @mcginty) wants to have a (post-merge) look I'd certainly wouldn't protest.

[mcginty]

I haven't tested it but this looks so much cleaner and more maintainable, great stuff.