ddtrace/opentelemetry: improve TestSpanContextWithStartOptions

[katiehockman]

What does this PR do?

Adds a check to the TestSpanContextWithStartOptions OTel test which verifies that the payload contains two spans.

Motivation

This test started failing while we were implementing partial flushing. We found that partial flushing was being triggered when it shouldn't have been, which led to the payload at the end only containing a single span, instead of 2 spans as would be expected. It took longer than necessary to realize that's why the test was failing though, so this just makes it easier to spot the root cause if this test fails in that way again.

Reviewer's Checklist

• Changed code has unit tests for its functionality.
• If this interacts with the agent in a new way, a system test has been added.

[pr-commenter]

Benchmarks

Benchmark execution time: 2023-06-27 19:01:25

Comparing candidate commit 027d923 in PR branch katie.hockman/otel-test with baseline commit 8dddd8b in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 27 metrics, 0 unstable metrics.