[5.1][New Test] Add loop iteration count signed/unsigned corner testcase

[tob2]

I did run into this issue recently – and think it could be a useful addition; it just needs to be properly packaged:

• Due to the int → unsigned int integer promotion, the i < 10u condition evaluates as false and the loop body is not executed.
• However, depending how the OpenMP loop iteration count is calculated, it might run 1010 times.

Since OpenMP 5.1, the following wording ensures that the OpenMP count and the C count are the same:

"If var has a signed integer type and the var operand of test-expr after usual arithmetic conversions has an unsigned integer type then the loop iteration count is computed from lb, test-expr and incr using an unsigned integer type corresponding to the type of var." (OpenMP 5.2, 4.4.2 OpenMP Loop-Iteration Spaces and Vectors and, likewise, OpenMP 5.1, 2.11.1 Canonical Loop Nest Form)

int
main ()
{
  int error = 0;
  #pragma omp for
  for (int i = -1000; i < 10u; ++i) {
     error = 1;
  }
  return error;
}