test: fix for xctl

Adjust the following test so that they take into acount xctl files
(i.e.  copy and/or delete them along with snap files):
 - wal_off/snapshot_stress
 - xlog/panic_on_wal_error
 - xlog-py/misc

Also, add a sanity check to xctl_begin_recovery() to make sure there's
not xctl file newer than the latest snap file.