wcfs: tests: Add test to exercies faulty client that does not reply to pin triggered by readPinWatchers

Levin writes:

    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 !18

Preliminary history:

    levin.zimmermann/wendelin.core@9d42efffCo-authored-by: Levin Zimmermann <levin.zimmermann@nexedi.com>

/discussed-on !18