testFlattenOpenOfficeForm() has differences

PDAcroFormFlattenTest.testFlattenOpenOfficeForm() now has differences, because flattening shows the appareances while the initial rendering doesn't.

We could either delete the test (we have testFlattenOpenOfficeFormFilled() instead which is fine), or change the test so that it refreshes the appearances (if needappearances is set) before the first rendering.