Rewrite complexapp test documentation

[lbussell]

What I did:

• Consolidated test documentation into run-tests-in-container.md instead of splitting it between there and complexapp/README.md
• Added section on running tests with Docker build using BuildKit's new(-ish) build output feature
• Re-ran example commands to get new output for .NET 9
• Removed the strict recommendation for running tests as an executable stage over running tests as part of Docker build. With BuildKit, they seem about equal to me.

Potential for improvement:

• I have not played with getting test output from the Docker build without buildkit. Theoretically you could build the test-output stage and then start the container and copy the test results out. However, if you don't have BuildKit you probably don't want to use this approach anyways. Since without BuildKit, the Docker builder won't skip stages in a multi-stage build.

[mthalman]

Didn't mean to approve yet. Changing my status.