For some reason, the height of a text line is 15px there and not 18px as
locally, so 400px high image still fit on the second page in the last
test. Make it higher to ensure that it doesn't.
Set the "printer" PPI explicitly for wxMemoryDC used in the test to
ensure that it's the same in all ports: currently wxGTK3 stands out
because it uses 72 DPI unlike wxMSW and wxGTK2, which use 96.
The standard margins, expressed in millimeters, could result in the
usable page space being much smaller than 1000px used for the DC size
when using higher DPIs, which means that the test checking that a 2400px
image took only 3 pages could fail, as it could require 4 of them in
this case.
Fix this by getting rid of the margins, as this should ensure that the
page height is exactly 1000px now, independently of the actual DPI.
