Avoid failure in VBA test when window is too small

The VBA test case "SplitRow" in window2.xls requires the spreadsheet
window to be large enough.  It looks like this has been discovered
previously and some of the assertions commented out with a note about
the window size requirement.  However we bumped into a failure in one of
the remaining assertions on one of our tinderboxes.

To resolve:

 - Maximize the window at the start of this test case

 - Skip the test if the window size is still too small

 - Re-enable assertions which had been commented out due to this issue.
   (Note that VBA which reads Window.SplitRow appears to be broken in
    LO, so I have left two assertions here commented out with a FIXME)

Change-Id: I5e87a360958f665457bbf8ec9c703a99794a62dc
Reviewed-on: https://gerrit.libreoffice.org/29110Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>