kunit: tool: Terminate kernel under test on SIGINT

kunit.py will attempt to catch SIGINT / ^C in order to ensure the TTY isn't
messed up, but never actually attempts to terminate the running kernel (be
it UML or QEMU). This can lead to a bit of frustration if the kernel has
crashed or hung.

Terminate the kernel process in the signal handler, if it's running. This
requires plumbing through the process handle in a few more places (and
having some checks to see if the kernel is still running in places where it
may have already been killed).

Reported-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Closes: https://lore.kernel.org/all/aaFmiAmg9S18EANA@smile.fi.intel.com/
Signed-off-by: David Gow <david@davidgow.net>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Tested-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>