Test projects cleanup

[mattjohnsonpint]

• Refactored to remove ASP.NET Core dependencies from Sentry.Testing, which unblocks MAUI Android Device Tests #1703
• Re-enables .NET Framework tests for ASP.NET Core 2.1. We were testing the OS using a deprecated technique, so these tests weren't actually running before - even on Windows. They are now, and restoring them also generated a missing API verification file.
• Updates all the tests to use the latest minor release of their various dependencies.
  • Note, we need to come up with a better way to stay evergreen with test dependencies. While we want our libraries to use minimum compatible versions, the tests should use latest.

#skip-changelog

[mattjohnsonpint]

Here's another case where we were checking incorrectly. If OS wasn't defined, then the tests didn't run for this target at all on any OS.

[mattjohnsonpint]

We can just include net461 directly now. The comment about the tests hanging on Mono appears to be not the case anymore. But note that it wasn't being included on Windows before because of the way we were checking.

Condition="'$(OS)' == 'Windows_NT'" is unsupported. The correct way is Condition="$([MSBuild]::IsOSPlatform('windows'))".