chore(ci): convert serverless slow import test to a subprocess test

[brettlangdon]

This test was messing with the global state of sys.modules. It was causing some modules to get unloaded causing them to get re-imported, which would cause issues.

Moving this test to a subprocess test will help isolate the behavior it is trying to test, and simplifies the test setup.

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.

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: brettlangdon/fix.serverless.test
Commit report: 08c0a31
Test service: dd-trace-py

✅ 0 Failed, 1326 Passed, 636 Skipped, 3m 14.92s Total Time

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-05-13 18:15:15

Comparing candidate commit 08c0a31 in PR branch brettlangdon/fix.serverless.test with baseline commit 2ac3363 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 209 metrics, 9 unstable metrics.