tests-util: Adding test to verify "allow-negative" flag

tests-util: Adding test to verify "allow-negative" flag

The standard string to integer conversion routines allow strings
with a leading "-" to indicate a negative number.
However, the returned value is always an unsigned value representing
the bit-pattern of this negative value.
Typically, this behaviour is NOT wanted and therefore the standard
behavior of the internal smb_strtoul(l) return an erros in such situations.
It can be enabled though by using the flag SMB_STR_ALLOW_NEGATIVE.
This test verifies the correct processing.

Signed-off-by: Swen Schillig <swen@linux.ibm.com>
Reviewed-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Christof Schmitt <cs@samba.org>