Rollup merge of #52453 - srijs:fix-52436, r=TimNN

Rollup merge of #52453 - srijs:fix-52436, r=TimNN

improve diagnostics for tests with custom return values

This is an attempt at getting the ball rolling to improve the diagnostics for test functions that return custom `impl Termination` values (see #52436).

An alternative could be to use `eprintln!`, but including this in the panic message felt nicely consistent with how failing test assertions would be reported.

Let me know what you think!