renameUser: add unit test

Like the previous change, this test added unit test for |renameUser|. To
make it happen, this commit introduces a virtual function
|executeUserModify| which is overridden in unit test (see previous
change for why we need such overriding).

Tested: unit test passed.

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: I230a8c12fd6c86d0fccfc00872eff647473c1e94
3 files changed