OpenMP test suite crash
The following tests crash with OpenMP (need to change parallelization const to 1)

This part of int matmul fallback

```Nim
  test "Integer Matrix-Matrix Multiplication fallback":
    ## TODO: test with slices
    let a = [[1,2,3],
             [4,5,6]].toTensor()

    let b = [[7,  8],
             [9, 10],
             [11,12]].toTensor()

    let ab = [[ 58, 64],
              [139,154]].toTensor()

    check: a * b == ab
```

Furthermore, there is a delayed "attempt to read from nil" that seem to be trigger and happens some tests later. Maybe due to GC collecting the data?

==> Todo: As OpenMP is deactivated for small matrices (<1000 elements), it's probably better to just test that the common operations work on 40x40 matrices in test_openmp as the small size might be the trigger.

==> Todo: a stress-test with large matrices that triggers OpenMP