Arranging for windows of particular dimensions in xdisp-tests

[Po Lu]

I've just fixed several hangs in redisplay arising from `display'
objects' being denied sufficient space by a line or wrap prefix, and
consequently continuation lines being produced in an endless cycle
inside move_it_to and display_line.  Needless to say, tests for such
bugs require that the windows where they execute be set to a particular
size, which, considering that ERT tests are expected to run in both
interactive sessions and in the initial frame, appears to be impossible.

Ideas?  Thanks in advance.