Refactor integration tests to prepare for proxy-server & agent skew testing

[tallclair]

This PR refactors the running of the test proxy-server & agent to prepare for running them as separate binaries, which will enable version skew testing.

This PR makes it so the server & agent are run with a RunnerInterface accessed through a global Framework. The runner interfaces returns a respective interface for accessing the testable surface of the server & agent.

The TestConcurrentClientRequest test needed to be rewritten, since it relied on internal implementation details. The new test is less deterministic, but should still reliably hit the same case being tested (and others).

First step towards #519

Next steps:

1. Implement External{ProxyServer,Agent}Runner, which takes an external binary to run for the server/agent, and accesses the measurements through prometheus metrics. Expose test flags to select a binary (or the in-tree version).
2. Implement a version_skew_test script that handles fetching target tagged versions, compiling, and running the tests.
3. Set up CI jobs for skew testing
4. Refactor client operations to enable external client testing via the test-client binary.
5. Implement ExternalClient, update version_skew_test to run tests with the external client

/assign @jkh52

I recommend reviewing this 1 commit at a time.

[tallclair]

Note to reviewers: I dropped ClientsCount here since it is not exposed by prometheus metrics. It wasn't actually being tested anyway, just used in the error message.

[jkh52]

Getting rid of this helps me in a Backend test improvement I started, thanks!

[jkh52]

/lgtm
/approve

Great contribution!

/hold

Hold in case you want to add that squash directive (some bits like t.Skip, FIXME, etc. show between commits).

[k8s-ci-robot]

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[tallclair]

/hold cancel

I think the t.Skip() is OK to have in an intermediate commit. It helps the commits to be focused, and ensures that tests still pass at each intermediate.