Add tests/README. ctdb_test_env and, therefore, run_tests can now be

Add tests/README.  ctdb_test_env and, therefore, run_tests can now be
run from the tests subdirectory... and most other sensible places.
Rename $CTDB_TEST_REMOTE_SCRIPTS_DIR to $CTDB_TEST_REMOTE_DIR since it
is now intented that this directory can contain test binaries too.
daemons_start() now passes a full path to the events.d directory when
starting ctdbd.  Also fixed the path to ctdbd.  41_ctdb_ban.sh and
42_ctdb_unban.sh fail immediately if they fail to select a test node.

Signed-off-by: Martin Schwenke <martin@meltin.net>
(This used to be ctdb commit 3ecce31d3a3f72b9fa851ac99291865c119e551e)