Make main.mysql_client_test non-great again

Re-enable main.mysql_client_test on all builders, because
at the moment we do not run any --big-test on buildbot
due to resource constraints.

A number of tests were declared big in
commit eeee1832
in an attempt to save resources on buildbot.

mysql-test/t/mysql_client_test.test

@@ -2,8 +2,6 @@
 -- source include/not_embedded.inc
 # need to have the dynamic loading turned on for the client plugin tests
 --source include/have_plugin_auth.inc
-# This test is slow on buildbot.
---source include/big_test.inc
 
 # Run test with default character set
 --source include/default_charset.inc