tests: net: sockets: tcp: Extend SO_RCVBUF/SO_SNDBUF tests

Add new test cases for SO_RCVBUF/SO_SNDBUF which verify that setting
those options actually affects the RX/TX window sizes at the TCP level.
Existing tests only verified whether the options can be set/read
correctly.

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