cbmc-cover/pointer-function-parameters test: negative numbers are ok

We need to cover three scenarios: `a` is `NULL`, or `*a` is 4, or `*a`
is some integer other than 4. The latter includes negative values.

Seen in
https://github.com/diffblue/cbmc/actions/runs/7740169274/job/21104572343?pr=8141.

Author: tautschnig

regression/cbmc-cover/pointer-function-parameters/test.desc

@@ -5,7 +5,7 @@ main.c
 ^Test suite:$
 ^(tmp(\$\d+)?=[^,]*, a=\(\(signed int \*\)NULL\))|(a=\(\(signed int \*\)NULL\), tmp(\$\d+)?=[^,]*)$
 ^(a=&tmp(\$\d+)?!0@1, tmp(\$\d+)?=4)|(tmp(\$\d+)?=4, a=&tmp(\$\d+)?!0@1)$
-^(a=&tmp(\$\d+)?!0@1, tmp(\$\d+)?=([012356789][0-9]*|4[0-9]+))|(tmp(\$\d+)?=([012356789][0-9]*|4[0-9]+), a=&tmp(\$\d+)?!0@1)$
+^(a=&tmp(\$\d+)?!0@1, tmp(\$\d+)?=([012356789]\d*|4\d+|-\d+))|(tmp(\$\d+)?=([012356789]\d*|4\d+|-\d+), a=&tmp(\$\d+)?!0@1)$
 ^EXIT=0$
 ^SIGNAL=0$
 --