Editing comment in test "test_trust_expire"
Editing comment in test "test_trust_expire"

The comment in test "test_trust_expire" was slightly changed for
more clarity. Now the comment describes in details why we shouldn't
set flag "subsecond" to True when we invoke timeutils.isotime(...).  

Change-Id: I2489f60c183b01c1d02367114b0ea404f60bc3cb

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