disable the test that doesn't work in MSAN

here MSAN complains that

==218853==WARNING: MemorySanitizer: use-of-uninitialized-value
   #0 0x7f84a77c60a3 in _gnutls_rnd_init /tmp/msan/lib/random.c:69:6
   #1 0x7f84a77c60a3 in gnutls_rnd /tmp/msan/lib/random.c:168:6

but the line lib/random.c:69 in gnutls-3.7.1 is

 69  	if (unlikely(!rnd_initialized)) {

and rnd_initialized is declared as

 40 static _Thread_local unsigned rnd_initialized = 0;

which apparently MSAN isn't happy with

mysql-test/suite/plugins/t/rpl_auth.test

@@ -1,3 +1,6 @@
+# MSAN doesn't like gnutls_rnd
+source include/not_msan.inc;
+
 if ($MTR_COMBINATION_ED25519) {
   if (!$AUTH_ED25519_SO) {
     skip No auth_ed25519 plugin;