Creating all the StagedUsers accounts is expensive. This change makes it
possible to reuse these test accounts across test methods within the
same class, saving about 10% of test execution time.
This change also saves a little time by adding reviewers to staged
changes in a single batch operation, rather than one API call at a time.
Other slight gains were achieved by having watchers watch All-Projects,
instead of watching a new project for every test method.
Change-Id: I413c8f98338be30dcf926a724333feb401c9aa52
af6146c03b