Add a way for tests to serialize and commit IPersistent objects. The
commit, for now, is implemented via calling external zodbtools/py to
commit raw data (see 7f14e2cb "go/zodb/fs1: tests: Factor-out commiting
a transaction via ZODB/py into common place). The data to commit is what
object serialization (see previous patch) gives.

For now there is no tracking of dirtiness. A test must explicitly list
objects it wants to save into database.

This functionality will soon be used to implement Persistent/DB/...
tests.