tracer: TestAgentIntegration: Clean up state for -count=2

[evanj]

What does this PR do?

This test currently fails when run more than once, such as with:

go test ./ddtrace/tracer -run=TestAgentIntegration -count=2

To fix it, clear the integration state after each test. Add a test that expects the default state of "no integrations" both at the start and at the end of the test function.

Additionally: these tests were starting an HTTP test server that appears to be unused. Remove that to simplify the test.

Motivation

I'm trying to run the tests with -count=10 to check for race bugs, but it currently does not work.

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.

For Datadog employees:

• If this PR touches code that handles credentials of any kind, such as Datadog API keys, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[pr-commenter]

Benchmarks

Benchmark execution time: 2023-10-16 08:05:47

Comparing candidate commit fb47bb6 in PR branch evan.jones/testagentintegration-clear-state with baseline commit 26fb2f4 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 39 metrics, 2 unstable metrics.

[ajgajg1134]

Looks good, thanks!!