Tidy up / redesign functional testing framework

[MichaelClerx]

Current code isn't well organised, needs a bit of tidying

[ben18785]

@MichaelClerx I'd really like to add more functional tests if we can sort out changepoint detection. (@abhidg -- would you be able to have a look at the changepoint detection? When we spoke, I was suggesting that we make some methods run on proportion passing rather than the raw scores.)

[MichaelClerx]

There's already tickets for those issues @ben18785 !

• Determine whether a test has passed/failed Determining whether a functional test has failed (changepoint) #780
• Change metric for optimiser changepoints to number of passes Change changepoint metric to number of passes per commit #906
• Other test ideas Implement some of these tests #843 Look at Geweke paper "Getting it right" #574

This issue is about tidying up the code

[abhidg]

@ben18785 I'll take a look at it this week