Test case for updating posts. Use CLOCK_TIMESTAMP instead of NOW.

- Add a new test case for updating posts. This also checks whether the
  'last_modified' field has been updated correctly.
- Use the CLOCK_TIMESTAMP SQL function instead of NOW. The former always
  returns the current time, the latter returns the time at the start of
  the transaction. Since all of our tests run in a single transaction
  that is rolled back after test execution, we need CLOCK_TIMESTAMP to
  be able to test changing timestamps.