test: medialibrary: check that thumbnailing works

The test creates a libvlc instance with a medialibrary and check that
the medialibrary can generate a mock thumbnail for a given mock:// URL
with different parameters.

Multiple improvements can be done from here: in particular, the
medialibrary will insist in loading its discovery modules, whereas we
only need to check whether the thumbnailing work in this current test.
Some work needs to be done to evaluate whether it's interesting enough
to cut those discovery modules out for this test or extend the scenario
to other cases like testing those discovery modules.

Regression testing from #27749
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