Update logic of mixed mode of dotnet test

[mariam-abdulla]

Relates to #45927

PR Overview

This PR updates the mixed mode logic for dotnet test by grouping unsupported test applications into a single message and bypassing individual messages in the loop.

• Consolidated the logging for non-testing platform applications into an early-return block
• Removed redundant using directives (System and System.IO)
• Adjusted the initialization flow for test applications

Reviewed Changes

File	Description
src/Cli/dotnet/commands/dotnet-test/MSBuildHandler.cs	Refactored test application initialization and removed some using directives to support the updated mixed mode logic

Copilot reviewed 15 out of 15 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (2)

src/Cli/dotnet/commands/dotnet-test/MSBuildHandler.cs:3

• Removing 'using System;' may cause compilation issues due to the usage of Environment.NewLine in the new code. Please re-add the directive if the type is not imported elsewhere in the file.

-using System;

src/Cli/dotnet/commands/dotnet-test/MSBuildHandler.cs:4

• Removing 'using System.IO;' may break the usage of Path.GetFileName in the new logging logic. Please ensure that Path is still accessible, or re-add the using directive.

-using System.IO;

[Youssef1313]

LGTM