After the previous patches, the MPTCP protocol can generate
fast-closes on both ends of the connection. Rework the relevant
test-case to carefully trigger the fast-close code-path on a
single end at the time, while ensuring than a predictable amount
of data is spooled on both ends.

Additionally add another test-cases for the passive socket
fast-close.

Signed-off-by: Paolo Abeni <[email protected]>
Message-Id: <ccf2c2fc0f7fdca9742c51b3fdc6bb6c5fabec5f.1662713367.git.pabeni@redhat.com>