selftests: defer: Introduce DEFER_PAUSE_ON_FAIL

selftests: defer: Introduce DEFER_PAUSE_ON_FAIL

The fact that all cleanup (ideally) goes through the defer framework makes
debugging of these commands a bit tricky. However, this also gives us a
nice point to place a hook along the lines of PAUSE_ON_FAIL. When the
environment variable DEFER_PAUSE_ON_FAIL is set, and a cleanup command
results in non-zero exit status, show a bit of debuginfo and give the user
an opportunity to interrupt the execution altogether.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/2a07d24568ede6c42e4701657fa0b738e490fe59.1757004393.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>