Refactor PreprocessorTest for better order handling

As mentioned by Mark, with the addition of Fileset:0
to many of the request lists, there are 6 orderings
possible. This required a little refactoring in order
to reduce the cut-n-pasting.
Add injected Fileset requests to the end of the list

Changes to spec.xml in this branch reverse when filesets
should be handled. With this commit, the operations which
handle filesets are:

 * /Fileset itself
 * /Image when in a DoAll
 * /Dataset by itself after all images
 * /Plate by itself after all images
 * and now /Dataset and /Plate in a DoAll at the end.
Order filesets last (Fix #11045, See #11054)

Rather than inject filesets at the first possible location
in the DoAll requests list, place them after all related
requests.