wcfs_test: Ensure faulty client is killed @readPinWatchers

This patch extends the test scope of 'test_wcfs_pintimeout_kill'. Before
this patch, the test only ensured that a client that does not
respond to pin requests during the initial watch request [1] is
killed. Now it also tests that a faulty client is killed when a block
is invalidated. Since there are no other situations where the WCFS
server sends pin requests to a client, the tests now cover all situations
where a faulty client might not respond. This patch therefore aims to
increase the security that WCFS is not blocked by a faulty client.

[1] See nexedi/wendelin.core!18