snapshot: Refactor snapshotxml2xml test

snapshot: Refactor snapshotxml2xml test

Upcoming changes want to separate out a post-parse massaging of
snapshots separate from parsing the XML, so as not to be dependent on
filtering out an ever-changing timestamp from the testsuite. Along the
way, this means we will want to add yet another conditional to the
snapshot xml2xml tests on whether to perform post-processing steps to
canned values. This will be easier to read if we consolidate all the
decisions into a flags variable, instead of adding yet another
boolean.

While at it, drop the redundant inout test of "noparent" (once is
enough).

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Cole Robinson <crobinso@redhat.com>