The problem was that xtradb has innodb_purge_threads default 1 (plain
innodb defaults to 0).

The test sets a special debug variable and relies on it to force
purge to happen. But when using background purge threads, this does
not work, the debug code is not made to handle this, so occasionally
the test times out waiting for the purge to occur.

Fix by explicitly setting innodb_purgee_threads=0 for this test.