fix nondeterministic hex_trace test

The cbmc/hex_trace test pattern relies on a particular value (0) for an
uninitialized local variable.  This removes the pattern.

See
https://github.com/diffblue/cbmc/actions/runs/6672135313/job/18269948126?pr=7979
as an exemplar where this was triggered.

Author: kroening

regression/cbmc/hex_trace/test.desc

@@ -4,7 +4,6 @@ main.c
 ^EXIT=10$
 ^SIGNAL=0$
 a=0 \s*\(0x0\)
-b=0ul? \s*\(0x0\)
 a=-100 \s*\(0xFFFFFF9C\)
 a=2147483647 \s*\(0x7FFFFFFF\)
 b=4294967294ul? \s*\(0xFFFFFFFE\)