[.NET 8 in-proc] Multi-target test projects

[liliankasem]

Issue describing the changes in this PR

Part of #10044

IMPORTANT: Currently, changes must be backported to the in-proc branch to be included in Core Tools and non-Flex deployments.

• Backporting to the in-proc branch is not required
  • Otherwise: Link to backporting PR
• My changes do not require documentation changes
  • Otherwise: Documentation issue linked to PR
• My changes should not be added to the release notes for the next release
  • Otherwise: I've added my notes to release_notes.md
• My changes do not need to be backported to a previous version
  • Otherwise: Backport tracked by issue/PR #issue_or_pr
• My changes do not require diagnostic events changes
  • Otherwise: I have added/updated all related diagnostic events and their documentation (Documentation issue linked to PR)
• I have added all required tests (Unit tests, E2E tests)

Additional Information

• Updated test projects to multi-target net6 and net8
• Updates ADO pipeline to run tests for both targets
• Updated unit test to separate out EGM from other unit tests (this is a temporary change whilst @brettsam works on multi-targeting for the EGM project and it's test - consider it out of scope for this PR).

[liliankasem]

@fabiocav - should I target this change into your clean branch for inproc8?

[liliankasem]

@fabiocav pipeline is all green except for the artifact build conflict due to having to rerun for a flakey test - is it okay if we bypass and merge (once we have approvals):

https://azfunc.visualstudio.com/Azure%20Functions/_build/results?buildId=165693&view=results

I wrote up a Teams thread about this pipeline issue for us to follow up on.