Previous change of have_debug_sync.inc broke non-debug builds.

Implement it in a different way that works on both release and debug
builds, and still uses --skip instead of --require.

mysql-test/include/have_debug_sync.inc

-if (`select @@debug_sync not like 'ON %'`) {
-  --skip Needs a debug_sync enabled
+if (`SELECT COUNT(*) = 0 FROM information_schema.session_variables WHERE
+      variable_name = 'debug_sync' AND variable_value LIKE 'ON %'`) {
+  --skip Needs debug_sync enabled
 }