Stub out dependency on subprocess in unit tests

Stub out the sheepdog Store dependency on subprocess communication
with collie.  This results in many unnecessary pipes being created
on POSIX systems.

For unit testing purposes, do not create the sheepdog store.  (There
are no unit tests for this functionality anyway.)

Change-Id: I54014a502891ae52bb567f3fae32e6ff0c74332a
Merge "Stub out dependency on subprocess in unit tests"