Reduce string-max-length on String.equals test

Execution time can vary depending on what the sat solver find as a
model at the refinement steps.
Observed execution time go from 0.2s to 7s.
Setting the limit to 100 limit these variations (maximum observed is
0.3s).

regression/jbmc-strings/StringEquals/test.desc

@@ -1,6 +1,6 @@
 CORE
 Test.class
---refine-strings --string-max-length 1000 --function Test.check
+--refine-strings --string-max-length 40 --function Test.check
 ^EXIT=10$
 ^SIGNAL=0$
 assertion at file Test.java line 6 .* SUCCESS