testsuite: repair read of uninitialized memory

Function ``test_backoff_time`` does not initialize ``delta``, and
``get_backoff_delta_msec`` then performs a read from uninitialized
memory with the ``!*delta`` expression.

Signed-off-by: Jan Engelhardt <jengelh@inai.de>
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>