Mock time for audit test 'test_sleeper'

The object auditor unit test, test_sleeper, needs to mock out time
to ensure run variations in the test environment do not skew the
results.

Change-Id: I6ef15e88d6f760bc805e4bb6a6270751984624ff
Closes-Bug: 1321019
Merge "Mock time for audit test 'test_sleeper'"