tests: net: tcp: Fix server tests

Fixing the bug with shadowed ret variable (and hence not returning
timeout error) reveled another bug in TCP server tests. Here, the
listening context was incorrectly used to receive new data, and since it
never arrived, the function timed out.

This commits fixes that issue, and limits the retransmission timeout to
ensure that Nagle's algorithm impact does not affect test timings.

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