Merge branch 'selftests/bpf: Add parallelism to test_progs'

Merge branch 'selftests/bpf: Add parallelism to test_progs'

Yucong Sun says:

====================

This patch series adds "-j" parelell execution to test_progs, with "--debug" to
display server/worker communications. Also, some Tests that often fails in
parallel are marked as serial test, and it will run in sequence after parallel
execution is done.

This patch series also adds a error summary after all tests execution finished.

V6 -> V5:
  * adding error summary logic for non parallel mode too.
  * changed how serial tests are implemented, use main process instead of worker 0.
  * fixed a dozen broken test when running in parallel.

V5 -> V4:
  * change to SOCK_SEQPACKET for close notification.
  * move all debug output to "--debug" mode
  * output log as test finish, and all error logs again after summary line.
  * variable naming / style changes
  * adds serial_test_name() to replace serial test lists.
====================

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>