Run innodb_wl6326_big only in debug builds

The test seems to deterministically fail on RelWithDebInfo builds
due to a timeout in wait_condition.inc.

According to Matthias Leich (the original author of the test),
the failure rate would reduce if we disabled the purge of
transaction history by setting innodb_force_recovery=2.

For now, let us run this stress test on debug builds only.

mysql-test/suite/innodb/t/innodb_wl6326_big.test

@@ -29,6 +29,10 @@
 # vardir on tmpfs : ~ 375
 # vardir on disk  : ~ 546
 --source include/big_test.inc
+# Possibly related to MDEV-16678, the test seems to deterministically fail on
+# non-debug builds. innodb_force_recovery=2 (disabling the purge of history)
+# would seem to help a little.
+--source include/have_debug.inc
 # We go with "--send" and "--reap" and that fails with the embedded server.
 --source include/not_embedded.inc
 # Its intentional to not take the risk that a run with valgrind times out.