tests: net: tcp: Fix a few issues with TCP tests

The net_context-based TCP tests required some adjustments/fixes:

* seq number was not incremented for the final ACK after FIN
* accepted net_context was not dereferenced (with net_context_put())
* net_context_put() should be used instead of net_tcp_put(), as it's the
  net_context that is allocated in the test (with net_context_get())
* out of order tests depend on each other, therefore it must be assured
  they're executed in the correct order

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>