Fix String test off-by-one error

This was incidentally exposed by the previous commit; due to comparing with < against a limit enforced
with > in the string solver, it always had the possibility not to abort as the test expects.

Author: smowton

regression/jbmc-strings/long_string/Test.java

@@ -30,7 +30,7 @@ public static void checkAbort(String s, String t) {
         String u = s.concat(t);
 
         // Filter out
-        if(u.length() < 67_108_864)
+        if(u.length() <= 67_108_864)
             return;
         if(CProverString.charAt(u, 2_000_000) != 'b')
             return;