Avoid that lone atoms which are part of a ring in one of the molecules become part of the MCS

[ptosco]

While looking at some of the test cases in #4019 I have noticed that currently the MCS does not do what is expected in this case:

mol1 = Chem.MolFromSmiles("C1CCCC1C")
mol2 = Chem.MolFromSmiles("C1CCCC1C1CCCCC1")

MolsToGridImage((mol1, mol2))

ps = rdFMCS.MCSParameters()
ps.AtomCompareParameters.CompleteRingsOnly = True
ps.timeout = 300
res = rdFMCS.FindMCS((mol1, mol2), ps)
res.queryMol

As AtomCompareParameters.CompleteRingsOnly was set to True, atom 5 should not be part of the MCS, as we have requested that only atoms which are part of a complete ring are included, and this is not the case.
The reason why this happens is that in #3695, when I implemented the AtomCompareParameters.CompleteRingsOnly parameter (in addition to the already existing BondCompareParameters.CompleteRingsOnly), I omitted to also set the AtomCompareParameters.RingMatchesRingOnly flag (as already happened for its BondCompareParameters.CompleteRingsOnly counterpart).
In fact, as AtomCompareParameters.CompleteRingsOnly implies BondCompareParameters.CompleteRingsOnly, a non-ring atom can never be matched to a ring atom as that would imply including an incomplete ring in the MCS.

With this change, the MCS returned in the above test case is cyclopentane, as expected.
I had to tweak the results of a few unit tests that I added in #3695 as the expected result did not include atom ring specifiers in the SMARTS string, as they should have.