PmCheck: Adjust recursion depth for inhabitation test

In #17977 (closed), we ran into the reduction depth limit of the typechecker. That was only a symptom of a much broader issue: The recursion depth of the coverage checker for trying to instantiate strict fields in the nonVoid test was far too high (100, the defaultMaxTcBound).

As a result, we were performing quite poorly on T17977. Short of a proper termination analysis to prove emptyness of a type, we just arbitrarily default to a much lower recursion limit of 3.

Fixes #17977 (closed).