Fix MTR test wsrep.variables_debug

The test was changing variable wsrep_provider dynamically,
but wsrep_provider was recently made read-only.

followup for ce3a2a688d

mysql-test/include/galera_no_debug_sync.inc

@@ -0,0 +1,9 @@
+--disable_query_log
+
+--let $galera_have_debug_sync = `SELECT 1 FROM INFORMATION_SCHEMA.GLOBAL_STATUS WHERE VARIABLE_NAME = 'wsrep_debug_sync_waiters'`
+
+--if ($galera_have_debug_sync) {
+	--skip Test requires Galera debug library with no debug_sync functionality
+}
+
+--enable_query_log