Fix untouchability test

This MR fixes #24938.  The underlying problem was tha the test for
"does this implication bring in scope any equalities" was plain wrong.

See
  Note [Tracking Given equalities] and
  Note [Let-bound skolems]
both in GHC.Tc.Solver.InertSet.

Then
* Test LocalGivenEqs succeeds for a different reason than before;
  see (LBS2) in Note [Let-bound skolems]

* New test T24938a succeeds because of (LBS2), whereas it failed
  before.

* Test LocalGivenEqs2 now fails, as it should.

* Test T224938, the repro from the ticket, fails, as it should.