tests: net: socket: tcp: Make test_vX_msg_waitall more robust

The close function on a TCP socket can return before the complete
socket has been closed, as there might be packets still in flight.
Add a wait at the end of the test so we are sure all sockets have
been closed before the next test starts.

Signed-off-by: Sjors Hettinga <s.a.hettinga@gmail.com>