test_prev_next_minute is needlessly slow

[maread99]

Because it blanket tests the edges of all sessions, the test_prev_next_minute is one of the slowest tests of the suite.

The test could be sped up considerably by focusing on the edges of sessions of Answers.session_blocks (as a number of other tests already do).

If anyone wants to get familiar with the test suite, this would be a good PR to take up (maybe start by having a look at the test inputs available from the Answers class and then at how the tests for other similar methods are implemented).