embedded tests: accept and ignore --tap argument

embedded tests: accept and ignore --tap argument

This makes them semi-command-line-compatible with a way we can
invoke the GLib-based tests to get more useful debug logs.

These tests still do not actually produce TAP output yet; I tried
implementing that, but it requires changing a lot of noise on stdout
to come out of stderr, and there was something weird going on with
subprocesses restarting the test numbering which will need further
investigation before making that change.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=93194
Reviewed-by: Ralf Habacker <ralf.habacker@freenet.de>