Add extra tests when born within 4 months and 1 day from state pension

As well as improving the test cases that were picked up, I've
simplified the conditional that was being used to work out the
next_node.
Merge pull request #477 from alphagov/more-tests-within-four-months-one-day-of-sp-date

Add extra tests when born within 4 months and 1 day from state pension