[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

flang/test/Driver/mlir-pass-pipeline.f90

@@ -1,6 +1,9 @@
 ! Test the MLIR pass pipeline
 
 ! RUN: %flang_fc1 -S -mmlir --mlir-pass-statistics -mmlir --mlir-pass-statistics-display=pipeline %s -o - 2>&1 | FileCheck %s
+
+! REQUIRES: asserts
+
 end program
 
 ! CHECK: Pass statistics report

flang/test/Fir/basic-program.fir

@@ -1,6 +1,8 @@
 // RUN: tco %s | FileCheck %s
 // RUN: tco %s --mlir-pass-statistics --mlir-pass-statistics-display=pipeline 2>&1 | FileCheck %s --check-prefix=PASSES
 
+// REQUIRES: asserts
+
 // Check that tco is working with a basic test.
 // Also check the passes in the default pipeline.