tests/lwip: Add a test for partial TCP reads.
tests/lwip: Add a test for partial TCP reads.

Add a new test to check the behavior of `sock_tcp_read` when more data
is available in the connection than the buffer passed. This test checks
issue #16124 as well as reading from multiple small network packets into
a single buffer.

authmillenon/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,miri64/RIOT,OlegHahm/RIOT,ant9000/RIOT,kYc0o/RIOT,kaspar030/RIOT,jasonatran/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,kYc0o/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,authmillenon/RIOT,kYc0o/RIOT,ant9000/RIOT,kaspar030/RIOT,OlegHahm/RIOT,kaspar030/RIOT,OTAkeys/RIOT,jasonatran/RIOT,authmillenon/RIOT,kaspar030/RIOT,miri64/RIOT,authmillenon/RIOT,jasonatran/RIOT,OlegHahm/RIOT,miri64/RIOT,OTAkeys/RIOT,kYc0o/RIOT,miri64/RIOT,jasonatran/RIOT,ant9000/RIOT,jasonatran/RIOT,ant9000/RIOT,ant9000/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,miri64/RIOT,kaspar030/RIOT,OlegHahm/RIOT