Make conda-store-server test port configurable

[kcpevey]

Related to https://github.com/conda-incubator/conda-store/issues/570

I have to use a non-default port (not 5000) on my mac for development. This PR allows the playwright tests to run on any port instead of hard-coding the port.

Description

This pull request:

• allows the playwright tests to run on a configurable port via env var
• creates a pytest fixture for the port
• updates the playwright test to use the new fixture

Pull request checklist

• [x] Did you test this change locally?
• [ ] Did you update the documentation (if required)?
• [ ] Did you add/update relevant tests for this change (if required)?

Additional information

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