Run integration tests against an external agent process

[tallclair]

This PR enables the agent executed by integration tests to be launched as a separate process from a prebuilt binary, chosen by the --agent-path flag.

Aside from the new agent runner, most of the changes in this PR are around the way metrics are tested, since they need to be scraped from the external process when used.

I will follow up with the external ProxyServer runner as a separate PR.

For #519

[tallclair]

/assign @jkh52

[jkh52]

Please fix the lint errors:

tests/framework/agent.go:136:9: G204: Subprocess launched with a potential tainted input or cmd arguments (gosec)
	cmd := exec.Command(r.ExecutablePath, args...)
	       ^
tests/framework/metrics.go:47:15: G107: Potential HTTP request made with variable url (gosec)
	resp, err := http.Get(metricsURL)
	             ^
pkg/testing/metrics/metrics.go:136:9: indent-error-flow: if block ends with a return statement, so drop this else and outdent its block (revive)
	} else {
		return promtest.ScrapeAndCompare(t.Endpoint, strings.NewReader(expected), fqName)
	}

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jkh52, tallclair

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[tallclair]

/label tide/merge-method-squash

[tallclair]

I fixed the security issues by excluding tests from the security linter. I think it's more likely to just create noise on test files.

[jkh52]

/lgtm

[jkh52]

arm64 build timeout

/retest