Add trace check in StringBuilder test for empty array

This tests would fail before the fix to the symex constant propagation
of strings.

Author: romainbrenguier

jbmc/regression/jbmc-strings/StringBuilderConstructors01/test-no-arg-fail.desc

@@ -1,8 +1,12 @@
 CORE
 StringBuilderConstructors01.class
---function StringBuilderConstructors01.noArgFail --max-nondet-string-length 1000
+--function StringBuilderConstructors01.noArgFail --max-nondet-string-length 1000 --trace
 ^EXIT=10$
 ^SIGNAL=0$
 ^VERIFICATION FAILED$
+dynamic_object2=\{\s*\}
 --
 ^warning: ignoring
+--
+The check for dynamic_object2 is to make sure the array is created empty and
+is not given arbitrary content before its final assignment.