Strengthen condition in TokenTest02 and reduce unwind

The check for VERIFICATION FAILED could match a null pointer check
rather than an assertion.

Author: romainbrenguier

jbmc/regression/jbmc-strings/TokenTest02/test.desc

@@ -1,8 +1,9 @@
 CORE
 TokenTest02.class
---max-nondet-string-length 1000 --unwind 15 --cp `../../../../scripts/format_classpath.sh . ../../../lib/java-models-library/target/core-models.jar`
+--max-nondet-string-length 1000 --unwind 6 --cp `../../../../scripts/format_classpath.sh . ../../../lib/java-models-library/target/core-models.jar`
 ^EXIT=10$
 ^SIGNAL=0$
 ^VERIFICATION FAILED$
+line 13 assertion at file TokenTest02\.java .*: FAILURE
 --
 ^warning: ignoring