Fixes #403.

* spot/twaalgos/cleanacc.cc (simplify_complementary_marks_here):
Assume that two colors are complementary in a broad sense, i.e., on is
present if (but not only if) the other is missing before applying
those rewritings.  Technically, this is usually not necessary, because
the changes done in #418 will cause such "covering-the-full-automaton"
pair of colors to be disjoint so complementary in a strict sense.  It
might make a difference (this is not tested) in presence of reused
colors which #418 cannot remove.  In any case, it's simply less code
to implement the broad sense.
* tests/python/toparity.py: Add test case from #403, which is
reduced to two states with recent optimizations.