Integration tests aren't compatible with rennovatebot

Rennovatebot isn't creating branches on the local repository, so the conformance tests which require this are failing. It's likely they will also fail for other external PRs as well.

https://github.com/GoogleCloudPlatform/functions-framework-php/blob/5b645eedc7508a16481c364e8cd209fb176d3e03/tests/conformance/prerun.sh#L25

We should instead direct the integration tests to point at the currently checked out source in the GitHub action's pipeline. Perhaps something like this: https://stackoverflow.com/questions/29994088/composer-require-local-package