Wait for the comment to be added before changing to another file

If the server is too slow, changing to a different file immediately
after sending a new comment but without waiting for the comment to be
shown for the original file could cause the new comment to be shown for
the current file instead.

This is, indeed, a bug in the comments. However, it is not possible to
test it reliably in the acceptance tests, as it depends on how fast the
server adds the message and how fast the client changes to a different
file; sometimes the test would fail and sometimes it would not.

Therefore, now it is waited for the comment to be added before changing
to another file, as in this case it can be reliably tested that changing
to a different file does not cause the comments from the previous file
to be shown in the current file (this was a different bug already fixed
and due to which this test was added in the first place).

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>