ctdb-tests: Use $CTDB_SCRIPTS_TESTS_BINDIR

Don't calculate this locally as _test_bin_dir.  Just calculate
top_dir, source script_install_paths.sh and use
$CTDB_SCRIPT_TESTS_BINDIR.

Don't bother sanity checking if TEST_BIN_DIR is set.  It will go away
soon.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>