12684: Use mock services in arvfile sparse write tests

12684: Use mock services in arvfile sparse write tests

Without these mocks, Jenkins seems to spend a lot of time retrying
requests—although weirdly, I don't see that in my own development
environment.

I believe the mocks were always intended to be used, since they're
instantiated and already used in other sparse write tests. To me this
looks like an oversight when the previous tests were adapted to write
new collections.

Arvados-DCO-1.1-Signed-off-by: Brett Smith <brett.smith@curii.com>