Fix broken tooltip tests #1577
Description
It seems that recently our tooltip tests are broken again.
I managed to debug the breakage in test_tooltip_std and it seems we hit an issue where we try to lower two different copies of core (which are practically the same but have different disambiguators), where one takes precedence and the second one is ignored.
This was originally done to make the definitions unambiguous when compiling regular and #[cfg(test)]-enabled views of the same crate.
However, the crate specifically refers to the identifier from the second crate, from where definitions are ignored, hence why we don't get a meaningful reference.
By always returning false in has_congruent_def we can "fix" (not sure how to go about it for a final solution) but I still don't know why we have breakage in test_tooltip (which still fails).