test-tool: add iSCSICHAP.Base64Oversize test

This is very similar to the previous Base64 CHAP test, except we
intentionally inject extra base64 characters into the CHAP_R field.
This should cause login to fail. On LIO this can trigger a buffer
overwrite reported at:
https://lore.kernel.org/target-devel/6a0a00f2.e1ea9722.1dc845.b85e@mx.google.com/

Signed-off-by: David Disseldorp <ddiss@suse.de>

test-tool/iscsi-test-cu.h

@@ -89,6 +89,7 @@ void test_iscsi_chap_simple(void);
 void test_iscsi_chap_invalid(void);
 #ifdef HAVE_LIBGNUTLS
 void test_iscsi_chap_base64(void);
+void test_iscsi_chap_base64_oversize(void);
 #endif
 
 void test_mandatory_sbc(void);