chore(dataflow): update conftest, make publisher subprocess a daemon

[davidcavazos]

Description

Fixes#6445, #6094

• Update conftest to use the newer version in the GPU samples
• Make the publisher subprocess a daemon so the main process doesn't wait indefinitely
• Other small overall improvements in the conftest

ℹ️ The large diff of the conftest is because I'm replacing the parent conftest with the more updated one from the gpu-examples. I deliberately separated the conftest updates so I could fix these issues in a separate PR without triggering the flaky tests.

Checklist

• I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• README is updated to include all relevant information
• Tests pass: nox -s py-3.6 (see Test Environment Setup)
• Lint pass: nox -s lint (see Test Environment Setup)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• Please merge this PR for me once it is approved.
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample