tests: Test pool cache and XML read performance separately
tests: Test pool cache and XML read performance separately

As I have feared, the cache is actually slower now than parsing of the
raw XML. On a large dataset (2265 components):
/Perf/Pool/ReadXML: 156 ms: OK
/Perf/Pool/ReadCache: 310 ms: OK
This is a valuable result for the ongoing refactoring of the AppStream
cache feature (which is based around XML and not GVariant, for ease of
maintenance).

ximion/appstream,ximion/appstream,ximion/appstream,ximion/appstream