selftest: reproducible: Set maximum report size

Diffoscope can end up running for a very long time if there are a lot of
changes. To put a limit on how long it can run, cap the maximum report
size at 250 MB by default.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 52d5f76f)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>