another test case for tickling StoreLockError

with this we can reproduce the error even while disabling access to
previous revision in tiddler_put or doing any kind of extra
serialization

note that the exception appears to occur in the third iteration:

    BEFORE ['foo'] hello world
    AFTER ['foo'] hello world

    BEFORE ['foo', 'bar'] lorem ipsum
    AFTER ['foo', 'bar'] lorem ipsum

    BEFORE None lorem ipsum\ndolor sit amet
    *boom*