selftests: net: terminate bkg() commands on exception

There is a number of:

  with bkg("socat ..LISTEN..", exit_wait=True)

uses in the tests. If whatever is supposed to send the traffic
fails we will get stuck in the bkg(). Try to kill the process
in case of exception, to avoid the long wait.

A specific example where this happens is the devmem Tx tests.

Reviewed-by: Joe Damato <joe@dama.to>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250811231334.561137-5-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/lib/py/utils.py

@@ -117,6 +117,7 @@ class bkg(cmd):
                          shell=shell, fail=fail, ns=ns, host=host,
                          ksft_wait=ksft_wait)
         self.terminate = not exit_wait and not ksft_wait
+        self._exit_wait = exit_wait
         self.check_fail = fail
 
         if shell and self.terminate:
@@ -127,7 +128,9 @@ class bkg(cmd):
         return self
 
     def __exit__(self, ex_type, ex_value, ex_tb):
-        return self.process(terminate=self.terminate, fail=self.check_fail)
+        # Force termination on exception
+        terminate = self.terminate or (self._exit_wait and ex_type)
+        return self.process(terminate=terminate, fail=self.check_fail)
 
 
 global_defer_queue = []