Create testing standard

[aaronrussellHO]

Content change

• [X] Please review the accessibility checks for content changes.

I can confirm:

• [X] Content does not include any code or configuration changes (excluding frontmatter information)
• [x] Content meets the content standards e.g. Writing a principle and Writing a standard
  • [x] TODO: Set Standard ID once approved before merging
• [X] Content is suitable to open source, i.e.:
  • Content does not relate to unreleased gov policy
  • Content does not refer to anti-fraud mechanisms
  • Content does not include sensitive business logic
• [X] Last updated date for content is correct

[daniel-ac-martin]

I think it would be good to call out a few things (possibly in the introduction):

• That testing should be automated, where possible
• That tests should NOT be silo'd to QAT and that developers should be able to iterate on the tests

[daniel-ac-martin]

Oh, do we also want to call out automated accessibility testing here? (Same issue as with security testing in that it can't catch everything.)