tests: assign proxy ports dynamically

[supervacuus]

I often encounter issues when running proxy integration tests because required ports are already in use, which leads to various errors.

This PR

• instructs mitmdump to acquire the test port from the OS
• parses the output of mitmdump (unbuffered!)
• and uses the assigned port number throughout the test
• example constructs the full URL from a port number we pass in via env SENTRY_TEST_PROXY_PORT

#skip-changelog

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[supervacuus]

Just saw that I only added @JoshuaMoelans because he initially wrote the proxy integration tests, but actually, anyone can review this. This is only a test convenience, in that it makes the test suite less likely to fail in environmental scenarios.