[flang][nfc] Add missing `REQUIRES: asserts` in a test

This change is required for release builds - see
https://reviews.llvm.org/D130185 for more context.

I'm sending this without a review as this is rather straightforward and
identical to the changes from https://reviews.llvm.org/D130185 (the test
updated in this patch wasn't yet in-tree when D130185 landed).