Expand testing chapter

[foster999]

• [x] #29
• [ ] #116
• [x] #128
• [x] #15
• [x] #141
• [x] #200
• [x] #111

DoD: All sub tasks have been completed.

[foster999]

Relates to #29

[RuSnook]

Links to #116

[joshlynchONS]

Links to #128

[foster999]

Provisional headings:

• Write tests as code
• Structure test files to match code structure
• Run tests against all code changes
• Write reproducible tests
• Use minimal and realistic test cases
• Test that new logic is correct (unit tests)
• Test that different parts of the code interact correctly (integration tests)
• Test that the whole system works (end to end tests)
• Test that user needs are met (user acceptance tests)
• Minimise repetition in test code (fixtures and parameterised tests)
• Write tests before writing logic (TDD)
• Write tests to ensure that bugs are fixed

Should mention in intro that testing assumes that code is packaged

[gisellerosetta]

See #146 for additional content

[gisellerosetta]

Unlinking so this issue won't close. (Hopefully)