Disable StackallocBlkTests test on Mono

[EgorBo]

Unblocks CI #84398

The following test is failing on Mono-Windows-x64:

[MethodImpl(MethodImplOptions.NoInlining)]
static void PoisonStack()
{
    var b = stackalloc byte[20000];
    Unsafe.InitBlockUnaligned(b, 0xFF, 20000);
    Consume(b);
}

[MethodImpl(MethodImplOptions.NoInlining)]
static void EnsureZeroed(byte* ptr, int size)
{
    for (int i = 0; i < size; i++)
    {
        if (ptr[i] != 0)
            throw new InvalidOperationException();
    }
}

[MethodImpl(MethodImplOptions.NoInlining)]
static void Consume(byte* ptr) { } // to avoid dead-code elimination

[MethodImpl(MethodImplOptions.NoInlining)]
public static Guid Test4096(out Guid g)
{
    const int size = 4096;
    PoisonStack();
    byte* p = stackalloc byte[size];
    EnsureZeroed(p, size);
    g = default;
    return default;
}

[ericstj]

@EgorBo does this run the failing test leg? Not seeing runtime (Build mono minijit Pri0 Runtime Tests Run windows x64 release)

[EgorBo]

/azp list

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[EgorBo]

@EgorBo does this run the failing test leg? Not seeing runtime (Build mono minijit Pri0 Runtime Tests Run windows x64 release)

No it wasn't triggered for some reason (I assume it doesn't have a trigger for changes in issues.targers) so I pushed a blank change to the test itself.