[UBSan] Increase robustness of tests

These UBSan tests assert the absence of runtime errors via `count 0`,
which means "expect no output".  This fails the test unnecessarily in
some environments (e.g., iOS simulator in our case).  Alter the test to
be a bit more specific and "expect no error" instead of "expect no
output".

rdar://65503408

Differential Revision: https://reviews.llvm.org/D85155