Disable broken string test

This currently breaks an assertion as described in the .desc file. It should be re-enabled
once string-refinement knows how to introduce sufficient indirection to overcome this.

Author: smowton

jbmc/regression/jbmc-strings/StartsWith/test_nondet.desc

@@ -1,8 +1,13 @@
-CORE
+KNOWNBUG
 Test.class
 --max-nondet-string-length 100 --unwind 10 --function Test.checkNonDet
 ^EXIT=10$
 ^SIGNAL=0$
 assertion at file Test.java line 66 .*: SUCCESS
 assertion at file Test.java line 69 .*: FAILURE
 --
+--
+The nondet test currently breaks the invariant at string_refinement.cpp:2162
+("Indices not equal..."), which checks that a given string is not used with
+multiple distinct indices. As the test checks s.substring(s, 1), two distinct
+indices are indeed used.