Make conda-store-server test port configurable

[kcpevey]

Related to #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

• 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

[netlify]

✅ Deploy Preview for kaleidoscopic-dango-0cf31d ready!

Name	Link
🔨 Latest commit	f8bec15
🔍 Latest deploy log	https://app.netlify.com/sites/kaleidoscopic-dango-0cf31d/deploys/6500ded9321a0300086c426b
😎 Deploy Preview	https://deploy-preview-571--kaleidoscopic-dango-0cf31d.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.