[dotnet ] fix dotnet v3, redux

[lucaspimentel]

Motivation

Follow-up from #2962 to fix tests for .NET 3.x release

Changes

• revert some changes from #2962
  • restore references to /opt/datadog/netcoreapp3.1/Datadog.Trace.dll
  • re-enable DD_TRACE_ENABLED test
  • leave auto-instrumentation disabled by default in parametric tests (CORECLR_ENABLE_PROFILING=0)
  • remove unnecessary reflection meant for Datadog.Trace.Manual.dll
  • leave reflection for GlobalSettings.DebugEnagled
• fix: use Release configuration in dotnet restore commands to avoid using the Datadog.Trace nuget package (this has been an issue for a few months, but was only recently discovered when we released tracer v3)

Workflow

1. ⚠️ Create your PR as draft ⚠️
2. Work on you PR until the CI passes (if something not related to your task is failing, you can ignore it)
3. Mark it as ready for review
   • Test logic is modified? -> Get a review from RFC owner. We're working on refining the codeowners file quickly.
   • Framework is modified, or non obvious usage of it -> get a review from R&P team

:rocket: Once your PR is reviewed, you can merge it!

🛟 #apm-shared-testing 🛟

Reviewer checklist

• [ ] If PR title starts with [<language>], double-check that only <language> is impacted by the change
• [ ] No system-tests internal is modified. Otherwise, I have the approval from R&P team
• [ ] CI is green, or failing jobs are not related to this change (and you are 100% sure about this statement)
• [ ] A docker base image is modified?
  • [ ] the relevant build-XXX-image label is present
• [ ] A scenario is added (or removed)?
  • [ ] Get a review from R&P team

[lucaspimentel]

• #2981
• #2978 👈
• main

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