MDEV-10986 sphinx.union-5539 and sphinx.sphinx fail in buildbot and outside

Do not include the suite into the default list until the tests are stable

storage/sphinx/mysql-test/sphinx/suite.pm

@@ -114,7 +114,7 @@ sub servers {
   )
 }
 
-sub is_default { 1 }
+sub is_default { 0 }
 
 ############# return an object ######################
 bless { };