ci(telemetry): expect observed test result

[emmettbutler]

Improves the reliability of the test suite by limiting the set of events retrieved in this telemetry test and expecting the occasionally-observed number of events.

The test agent session is shared between many tests, and it's not reliable at resetting itself between tests. The more complete and permanent fix to these issues is to make the test agent session reliable.

Checklist

• Change(s) are motivated and described in the PR description
• Testing strategy is described if automated tests are not included in the PR
• Risks are described (performance impact, potential for breakage, maintainability)
• Change is maintainable (easy to change, telemetry, documentation)
• Library release note guidelines are followed or label changelog/no-changelog is set
• Documentation is included (in-code, generated user docs, public corp docs)
• Backport labels are set (if applicable)
• If this PR changes the public interface, I've notified @DataDog/apm-tees.
• If change touches code that signs or publishes builds or packages, or handles credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.

Reviewer Checklist

• Title is accurate
• All changes are related to the pull request's stated goal
• Description motivates each change
• Avoids breaking API changes
• Testing strategy adequately addresses listed risks
• Change is maintainable (easy to change, telemetry, documentation)
• Release note makes sense to a user of the library
• Author has acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment
• Backport labels are set in a manner that is consistent with the release branch maintenance policy

[datadog-dd-trace-py-rkomorn]

Datadog Report

Branch report: emmett.butler/telemetry-accc-flake
Commit report: 2bd0712
Test service: dd-trace-py

✅ 0 Failed, 203 Passed, 223 Skipped, 20.5s Total duration (1m 6.68s time saved)