Disable test for load-save identity

This test appeared to be passing for the wrong reason, it's actually not
appropriate for the current implementation. The serialised data contains
values of pointers to heap-allocated buffers. There is no reason these should
be identical after a load-save pair. They just happened to be identical when I
first ran the test due to the place of session_free() in the test code and the
fact that the libc's malloc() reused the same buffers. The test no longer
passes if other malloc() implementations are used (for example, when compiling
with asan which avoids re-using the buffer, probably for better error
detection).

So, disable this test for now (we can re-enable it when we changed how
sessions are serialised, which will be done in a future PR, hence the name of
the dummy macro in depends_on). In the next commit we're going to add a test
that save-load is the identity instead - which will be more work in testing as
it will require checking each field manually, but at least is reliable.

tests/suites/test_suite_ssl.function

@@ -663,7 +663,7 @@ exit:
 }
 /* END_CASE */
 
-/* BEGIN_CASE */
+/* BEGIN_CASE depends_on:MBEDTLS_SEE_FUTURE_PR */
 void ssl_serialise_session_load_save( int ticket_len, char *crt_file )
 {
     mbedtls_ssl_session session;