selftests/bpf: Make tcp_rtt test more robust to failures

Switch to non-blocking accept and wait for server thread to exit before
proceeding. I noticed that sometimes tcp_rtt server thread failure would
"spill over" into other tests (that would run after tcp_rtt), probably just
because server thread exits much later and tcp_rtt doesn't wait for it.

v1->v2:
  - add usleep() while waiting on initial non-blocking accept() (Stanislav);

Fixes: 8a03222f508b ("selftests/bpf: test_progs: fix client/server race in tcp_rtt")
Signed-off-by: Andrii Nakryiko <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>
Reviewed-by: Stanislav Fomichev <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

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