Add .NET 7 Unit Test Build Target

[tippmar-nr]

Describe the story, please be clear on scope of the story.

Unit tests currently run only against a .NET Framework 462 build target, but we have numerous bits of code that are excluded from .NET Framework builds and thus are not covered by unit tests at present.

Add a .NET 7 build target to the Core.UnitTest project and modify existing tests so that they pass under both TFMs. A POC branch for this change is at https://github.com/newrelic/newrelic-dotnet-agent/tree/poc/net-7-unit-tests - in that branch, there are around 100 failing unit tests that need to be investigated. If a test is only applicable to .NET Framework, exclude it from .NET 7 builds. Otherwise, make changes as needed to ensure that the tests pass.

Acceptance Criteria

Unit Tests should execute against a .NET Framework build target as well as a .NET 7 build target. All unit tests should pass.

Estimates

Please provide initial t-shirt size
M

[workato-integration]

https://issues.newrelic.com/browse/NEWRELIC-9397

[chynesNR]

Completed in: #1778