[FIX] mail: tests: 'load more messages on scroll' works with phantomjs

Before this commit, this test was failing due to phantomjs assuming that
there could be more messages to display on the screen.

The issues comes from the bad support of flexbox with PhantomJS, and this
test broke when we changed a DIV display with 'flex' instead of 'block'.

For instance, this is the expected values:

    thread clientHeight:   960
    thread scrollHeight:  1629
    message clientHeight:   50

With 30 messages, 50 x 30 = 1500 < 1629 and 960 < 1629, so there are
enough messages loaded with this screen and we are not at the top of
the thread (this is the case when clientHeight = scrollHeight).

However, with phantomJS we have the following values:

    thread clientHeight:   944
    thread scrollHeight:   944
    message clientHeight:  259

As a result, with 30 messages, 50 x 30 = 1500 > 944 (looks like
messages do not vertically collide ?) and 944 = 944 so it assumes
we are at the top of the thread, so it fetches for more messages.

This commit fixes this test by enforcing a special rendering of the
thread for phantomJS.

Unfortunately, we can't simply make the hack on the test only. We
have to make discuss and thread aware of this because they should
adapt their rendering right away

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