tests: net: socket: tcp: Make test_open_close_immediately 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.
Modify the test to wait briefly (less then the retransmit time), before
counting the number of still open net_contexts. This makes the test
outcome not dependent on the scheduling order of the different tasks.

Secondly the test actually checks the number of open contexts to be zero
so there is no need to wait for any open contexts to still close.

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