selftests/bpf: Make seen_tc* variable tests more robust

Martin reported that on his local dev machine the test_tc_chain_mixed() fails as
"test_tc_chain_mixed:FAIL:seen_tc5 unexpected seen_tc5: actual 1 != expected 0"
and others occasionally, too.

However, when running in a more isolated setup (qemu in particular), it works fine
for him. The reason is that there is a small race-window where seen_tc* could turn
into true for various test cases when there is background traffic, e.g. after the
asserts they often get reset. In such case when subsequent detach takes place,
unrelated background traffic could have already flipped the bool to true beforehand.

Add a small helper tc_skel_reset_all_seen() to reset all bools before we do the ping
test. At this point, everything is set up as expected and therefore no race can occur.
All tc_{opts,links} tests continue to pass after this change.

Reported-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20231006220655.1653-7-daniel@iogearbox.net
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>