syncbase: Refactor and expand Watch tests.
syncbase: Refactor and expand Watch tests.

Refactored Watch tests to reduce inline boilerplate and expanded
tests to cover trickier cases.
Also fixed Watch error when checking ACLs of a Collection that no
longer exists. Expanded test to cover this case.

MultiPart: 1/2
Change-Id: I7cedf3350645cb1bb55a7444499d32968755ff3b

vanadium/go.v23,vanadium/go.v23,vanadium/go.v23