Add test for instanceof String

This checks the bug with class identifier of nondet strings was fixed.

regression/jbmc-strings/instanceof/Test.java

@@ -0,0 +1,9 @@
+public class Test {
+
+    public static String check(String s) {
+        if (s == null)
+            return "null";
+        assert(s instanceof String);
+        return "non-null";
+    }
+}

regression/jbmc-strings/instanceof/test.desc

@@ -0,0 +1,8 @@
+CORE
+Test.class
+--refine-strings --string-max-length 1000 --function Test.check
+^EXIT=0$
+^SIGNAL=0$
+assertion at file Test.java line 6 .* SUCCESS$
+^VERIFICATION SUCCESSFUL$
+--