I work on a codebase (LibreOffice) that has seen those days (and I worked through them on other projects, but thankfully without needing cross-platform support).

I note that there is some sharing of test-suites between the different compiler vendors, certainly they occasionaly run their compilers against some of each others test.

But a central shared resource that is updated by multiple people to catch edge cases would be first prize.

(And hopefully, seeded by the proposal authors, to act as a useful starting point)