[dotnet] Return back AlertsTest on .net 4.8

[nvborisenko]

Jim already fixed this issue, we can return tests back.

Motivation and Context

Getting more test coverage.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the contributing document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.