To make these helpers as least invasive as possible simply initialize
the options with a getopt parser and let the control flow be steered
with a simple helper which gets the subtest name as an argument.

The only tricky part for using it is that the subtest check helper
doubles up as the conduit to enumerate tests (and in that mode
prevents any test from being run). It is therefore important that
nothing gets printed to stdout outside of these checks.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>