net: tcp2: Use net_context to access TCP data

In order to properly remove struct tcp and its data, net_context
needs to be supplied as k_timer user data. Modify net_tcp_unref()
to take a net_context argument when removing, and add ifdefs
around code that after this will only be used by TCP testing
functionality.

Signed-off-by: Oleg Zhurakivskyy <oleg.zhurakivskyy@intel.com>