Make `update_test` compatible with nose

Nose's default test-finder will look for tests matching 'test*' and '*test'.

This means that nose will treat `update_test` as a test itself and attempt to
call it directly, not what we want!

Fortunately, nose has a rule to ignore tests with leading underscores, so we
can just prefix with that, as we've done elsewhere.

Change-Id: I81943241948e60989c99d5ae17cd4b5c2b1a7a23
Merge "Make `update_test` compatible with nose"