[patch v1] malloc: add tst-mxfast to hugetlb exclusion list

[Wilco Dijkstra]

Hi DJ,

> tst-mxfast needs GLIBC_TUNABLES to be set to its own value.

LGTM.

Reviewed-by: Wilco Dijkstra  <Wilco.Dijkstra@arm.com>

Cheers,
Wilco