Enable mxs1778_causal_reads with 10.3.8

The MariaDB 10.3.8 release fixes the use of last_gtid as a value for
session_track_system_variables. This means that the test can be enabled if
the backends are new enough.

maxscale-system-test/mxs1778_causal_reads.cpp

@@ -8,9 +8,7 @@
 
 int main(int argc, char** argv)
 {
-    printf("THIS TEST IS DISABLED UNTIL A SERVER VERSION WITH THE FIX IS RELEASED\n");
-    return 0;
-
+    TestConnections::require_repl_version("10.3.8");
     TestConnections test(argc, argv);
     const int N_QUERIES = 100;