When test_tcpbpf_user runs it complains that it can't find files
tcp_server.py and tcp_client.py.

Rework so that tcp_server.py and tcp_client.py gets installed, added them
to the variable TEST_PROGS_EXTENDED.

Fixes: d6d4f60c ("bpf: add selftest for tcpbpf")
Signed-off-by: NAnders Roxell <anders.roxell@linaro.org>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>