Cleanups in displayObject unit tests

Unit tests should be
- self-documenting
- reduced to test exactly what is intended to be tested, nothing more
- not repeat things tested previously
- as simple and clear as possible
Replace invalid multiplications by 0.05 with division by 20

To go back from twips, the value has to be divided by 20. Replacing that with 0.05 is invalid, because `/20` isn't reciprocal: for some values, it produces different results. Because floating point math.
Merge pull request #1384 from tschneidereit/nat

Fix twips-to-pixels rounding issues