[flang][nfc] Add missing `REQUIRES: asserts` in tests

Tests that use `--mlir-pass-statistics-display=` from MLIR require the
following condition to hold: (extracted from LLVM's Statistics.h):
```
  #define LLVM_ENABLE_STATS 1
```
This is normally enforced with `REQUIRES: asserts`. This patch updates
relevant Flang tests accordingly.

For "Release" builds (with assertions disabled), the affected tests will
be failing without this change.

Differential Revision: https://reviews.llvm.org/D130185