Add verification test for String.equals

We add tests which compare the internal String.equals with a reference
implementation which loops over the characters of the string.

Author: romainbrenguier

regression/jbmc-strings/StringEquals/test_verify.desc

@@ -0,0 +1,9 @@
+KNOWNBUG
+Test.class
+ --string-max-input-length 20 --string-max-length 100 --unwind 30 --function Test.verify
+^EXIT=0$
+^SIGNAL=0$
+assertion at file Test.java line 62 .* SUCCESS
+--
+--
+Our current implementation does not handle the null case

regression/jbmc-strings/StringEquals/test_verify_non_null.desc

@@ -0,0 +1,7 @@
+CORE
+Test.class
+ --string-max-input-length 20 --string-max-length 100 --unwind 30 --function Test.verifyNonNull
+^EXIT=0$
+^SIGNAL=0$
+assertion at file Test.java line 50 .* SUCCESS
+--