Muon(Combined)Config: fix unmerged CAs

This fixes some unmerged (Combined)Muon CAs exposed by !61689 (merged)

Tested locally with make test, i.e. the unit tests.

I hope this won't introduce FT0-violating changes, but I'm going to let the CI test that.

Incidentally (cc @wlampl, I'm wondering if there is a reason not to modify ComponentAccumulator to return itself in some methods. For example, it surprised me that this results in a dropped CA:

    return result.merge(MyToolCfg(flags))

You need to do it in two steps e.g.:

   result,merge(MyToolCfg(flags))
   return result

Similarly, see !61772 (merged), which could be made more compact if setPrivateTools returned self

(I know Guido van Rossum was anti chaining in the past but personally I don't really understand why)

cc @fwinkl @tstreble