tests: New function ctdb_set_pnn() to change PNN

tests: New function ctdb_set_pnn() to change PNN

ctdb_get_pnn() incorrectly caches to the same file regardless of what
node is selected via FAKE_CTDB_PNN.

Instead, set the PNN using new function ctdb_get_pnn(), which also
makes CTDB_VARDIR point to a node-specific subdirectory.  This means
that ctdb_get_pnn() will correctly cache to the node-specific
directory.

Fake tickle and TDB files/directories used by the ctdb stub need to be
the same across all PNNs, so change these to use
$EVENTSCRIPTS_TESTS_VAR_DIR instead of node-specific $CTDB_VARDIR.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>
(Imported from commit af93ae1a540003824b32301d3c9f09c713f1fa7a)