selftests/bpf: test_progs: fix client/server race in tcp_rtt

This is the same problem I found earlier in test_sockopt_inherit:
there is a race between server thread doing accept() and client
thread doing connect(). Let's explicitly synchronize them via
pthread conditional variable.

v2:
* don't exit from server_thread without signaling condvar,
  fixes possible issue where main() would wait forever (Andrii Nakryiko)

Fixes: b5587398

 ("selftests/bpf: test BPF_SOCK_OPS_RTT_CB")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>