torture:charset: use < and > assertions for strncasecmp_m

torture:charset: use < and > assertions for strncasecmp_m

strncasecmp_m is supposed to return a negative, zero, or positive
number, not necessarily the difference between the codepoints in
the first  character that differs, which we have been asserting up to
now.

This fixes a knownfail on 32 bit.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15625

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>