selftests: net: amt: indicate progress in the stress test

Our CI expects output from the test at least once every 10 minutes.
The AMT test when running on debug kernel is just on the edge
of that time for the stress test. Improve the output:
 - print the name of the test first, before starting it,
 - output a dot every 10% of the way.

Output after:

  TEST: amt discovery                                                 [ OK ]
  TEST: IPv4 amt multicast forwarding                                 [ OK ]
  TEST: IPv6 amt multicast forwarding                                 [ OK ]
  TEST: IPv4 amt traffic forwarding torture               ..........  [ OK ]
  TEST: IPv6 amt traffic forwarding torture               ..........  [ OK ]

Reviewed-by: Taehee Yoo <ap420073@gmail.com>
Link: https://patch.msgid.link/20250403145636.2891166-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>