MDEV-8298 sys_vars.all_vars fails in -DWITH_WSREP=OFF build

Disable sys_vars.all_vars test. There's no need to write
boilerplate tests for every new system variable, our
INFORMATION_SCHEMA.SYSTEM_VARIABLES and sysvar_* tests
are now used to show all basic properties of all variables.

mysql-test/suite/sys_vars/disabled.def

@@ -12,3 +12,4 @@
 
 innodb_flush_checkpoint_debug_basic: removed from XtraDB-26.0
 table_open_cache_instances_basic: no such variable in MariaDB
+all_vars: obsolete, see sysvars_* tests