Add LMDB KDB module tests

Add LMDB KDB module tests

Add a new k5test function multidb_realms() which yields a realm using
BDB and a realm using LMDB.  Run the tests in t_dump.py, t_iprop.py,
the lockout tests in t_policy.py, and kdbtest (in t_kdb.py) with both
databases.

If K5TEST_LMDB is set in the environment, use LMDB for all Python
tests and all tests in tests/dejagnu, with two exceptions
(t_unlockiter.py and t_kdb_locking.py) which test BDB-specific
behavior.  To support those exceptions, add the "bdb_only" realm
initialization flag.

In t_iprop.py, be sure to properly clean up kpropd daemons, using a
new k5test realm method stop_kpropd() to stop a kpropd daemon started
with start_kpropd() and remove it from the realm's list.

ticket: 8674