osmesa/test: Clear the stencil bits in the depth test.

osmesa/test: Clear the stencil bits in the depth test.

Since the depth buffer starts out as a malloc, and we weren't clearing it,
you could get undefined values in your top 8 bits.  This should fix
intermittent failures of the depth test.

(Sadly, valgrind wasn't catching this, presumably because the 32-bit value
there *is* written, just some bits are left undef)

Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/1243>