net: tcp: Explicitly wait for context closure instead of simply waiting

Previously the test waited for the worst case time till all sockets
where closed, this causes the test to wait for a long unnecessary time.
Secondly it does not ensure the sockets are actually closed before
starting the next tests.

For tests that are not conducted in user mode, wait until all the contexts
are close for a maximum time. If the contexts are not closed in time raise
an explicit assertion.

Signed-off-by: Sjors Hettinga <s.a.hettinga@gmail.com>