Fix exception with secretutils

1. There are some problems about the test method.
 problem 1:
  Unit tests may not cover our function, it depends on the python version
  that performed the test.
 problem 2:
  when using function 'constant_time_compare(first, second)', 'first' and
  'second' params are usually HMAC digest values, it is not appropriate to
  use utf-8 encoded values as mock data.

2. The previous commit `f1d332a` lead into a bug, but due to the problem 1
   and the problem 2, we did not find out the error.

Change-Id: I1c29bfe69f8eda60f3c5caaf3e5447dd5b69b108
Closes-Bug: #1772851

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