convert metadata test dumps to YAML for easy comparison

convert metadata test dumps to YAML for easy comparison

When making code changes related to the metadata parsing, it is useful to
see how the internal format has changed by seeing the differences in the
dump files.  Those files are currently in the binary .pickle format.  This
just straight converts them to YAML, which is a text format, so that normal
diff tools work to see changes.

The dump files are named .yaml instead of .yml since .yml is used for hand-
edited YAML files for fdroiddata/metadata, while these dump files here are
a human readable form of a Python pickle.