Add note on running tests in parallel to testing documentation

[kate-osborn]

Update our developer testing doc to cover running tests in parallel.

Doc should mention:

• In general, all tests should be parallelizable
• To run a go test in parallel, you must add t.Parallel() to every test and every subtest
• Ginkgo tests automatically run in parallel.
• If the component under test requires sequential tests -- for example, the change processor tests -- you can run the test sequentially by using an Ordered container for ginkgo tests or omitting the t.Parallel() from a go test or subtest. In these cases, you must add a comment to the test explaining why it cannot be run in parallel.

doc: https://github.com/nginxinc/nginx-gateway-fabric/blob/main/docs/developer/testing.md