ctdb-tests: Avoid creating files in /tmp.

ctdb-tests: Avoid creating files in /tmp.

Temporary test data should all go somewhere under TEST_VAR_DIR instead
of in the global /tmp.  The existing mktemp could be changed so the
data goes into the test directory but mktemp is overkill in this case.

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