Refactor feature tests and add combination test

- Split read api tests out into more specific feature tests
- Add feature test for combinations of multiple query parameters

@cpawar
@robyoung
Add newline

Fix my OCD.
Add log directory

Tests failed otherwise, since the directory doesn't exist where
it tries to create the log files.
Constrain healthcheck HTTP methods
Add test for _status allowed methods
Merge branch 'master' into refactored_feature_tests

Feature tests have moved into sub directories to better organise them.
Merge pull request #39 from alphagov/refactored_feature_tests

Refactor feature tests and add combination test
Pulling across data gen script from testing branch
Merge branch 'master' into fill-gaps-in-period-query
Enable feature test for query with data gaps

@robyoung
@maxfliri