[ios] Fix FollowManagementViewController test

Test DidSelectRow fails in official device builders. The test uses a
class mock object that is outliving the test case. I debugged the test
and it seems like the instances returned by the mocked class are
retained by some object (probably UIKit). Since the mock class object
returns itself when a new instance is created (I don't totally
understand why), the mock class object is retained across tests which
leads to crashes.

The fix consists of making the mock class object returning a partial
mock of an instance of the mocked class. Debugging shows that using
this approach doesn't retain the mock class object after the test
finishes. This means that the mocked class is restored to its
original state.

Bug: 339250378
Change-Id: Ia57ec4065722e8f566e870c8aa1a516eb562298f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5530774
Reviewed-by: Daniel White <danieltwhite@google.com>
Owners-Override: Ernesto Izquierdo Clua <eic@google.com>
Commit-Queue: Ernesto Izquierdo Clua <eic@google.com>
Cr-Commit-Position: refs/heads/main@{#1300250}