Powerpoint writer tests: New test framework for pptx.

Previously we had tested certain properties of the output PowerPoint
slides. Corruption, though, comes as the result of a numebr of
interrelated issues in the output pptx archive. This is a new
approach, which compares the output of the Powerpoint writer with
files that we know to (a) not be corrupt, and (b) to show the desired
output behavior (details below). This commit introduces three tests
using the new framework. More will follow.

The test procedure: given a native file and a pptx file, we generate a
pptx archive from the native file, and then test:

1. Whether the same files are in the two archives

2. Whether each of the contained xml files is the same. (We skip time
   entries in `docProps/core.xml`, since these are derived from IO. We
   just check to make sure that they're there in the same way in both
   files.)

3. Whether each of the media files is the same.

Note that steps 2 and 3, though they compare multiple files, are one
test each, since the number of files depends on the input file (if
there is a failure, it will only report the first failed file
comparison in the test failure).

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