Add new section for running tests locally
Add new section for running tests locally

This expands on the original one-liner and also pulls in some of
the content from the duplicate doc in Publishing API [^1].

I've added an extra example for running against a branch of API
Adapters, as documented in [^2].

[^1]: https://github.com/alphagov/publishing-api/blob/dd8dd9232d3cbf33b8945fdd898ebe80d7dcfcf6/docs/pact_testing.md#running-the-pacts-in-development
[^2]: https://github.com/alphagov/gds-api-adapters/pull/1049

alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs