Correctness fix for DelegatingEqualityComparer in Test Suite

The code is likely influenced by a classes of the same name in SemanticComparison, which don't [directly/ at present] fall into the same trap as the default ctors assign a [correctly] conservative default impl.

Will leave it to others to consider whether that code should bear warning comments or simply not expose an OnGetHashCode and have a safe impl with a comment as to why (the non-generic variant is the only consumer of OnGetHashCode and the generic one seems to have escaped that scrutiny).

Just noticed typo: assumtions
Will fix iff any followups