Set string-max-length in indexOf test

Without this, the solver sometimes ends up with a difficult instance for minisat.

Author: romainbrenguier

regression/jbmc-strings/StringIndexOf/test.desc

@@ -1,6 +1,6 @@
 CORE
 Test.class
---refine-strings --function Test.check --unwind 4 --string-max-input-length 3 --java-assume-inputs-non-null
+--refine-strings --function Test.check --unwind 4 --string-max-input-length 3 --string-max-length 100 --java-assume-inputs-non-null
 ^EXIT=0$
 ^SIGNAL=0$
 ^VERIFICATION SUCCESSFUL$