tests: ipc: check deadlock-like situation due to mixing priorities

Compared to the outer world, libqb brings rather unintuitive approach
to priorities within a native event loop (qbloop.h) -- it doesn't do
an exhaustive high-to-low priorities in a batched (clean-the-level)
manner, but rather linearly adds a possibility to pick the handling
task from the higher priority level as opposed to lower priority ones.

This has the advantage of limiting the chances of starvation and
deadlock opportunities in the incorrectly constructed SW, on the other
hand, it means that libqb is not fulfilling the architected intentions
regarding what deserves a priority truthfully, so these priorities are
worth just a hint rather than urgency-based separation.

And consequently, a discovery of these deadlocks etc. is deferred to
the (as Murphy's laws have it) least convenient moment, e.g., when
said native event loop is exchanged for other (this time priority
trully abiding, like GLib) implementation, while retaining the same
basic notion and high-level handling of priorities on libqb
side, in IPC server (service handling) context.

Hence, demonstration of such a degenerate blocking is not trivial,
and we must defer such other event loop implementation. After this
hassle, we are rewarded with a practical proof said "high-level
handling [...] in IPC server (service handling) context" contains
a bug (which we are going to subsequently fix) -- this is contrasted
with libqb's native loop implementation that works just fine even
prior that fix.

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>