[PROTON-2793] [proton-dotnet] Fix test code build warnings under .NET 8

• Type: Task
• Status: Resolved
• Priority: Trivial
• Resolution: Fixed
• Affects Version/s: proton-dotnet-1.0.0-M9
• Fix Version/s: proton-dotnet-1.0.0-M10
• Component/s: proton-dotnet
• Assignee: Timothy A. Bish
• Reporter: Timothy A. Bish
• Created: 20/Feb/24 23:26
• Updated: 20/Feb/24 23:29
• Resolved: 20/Feb/24 23:29