Make dump output tests line-ending agnostic

[smurtdev]

Summary

• Use xUnit's ignoreLineEndingDifferences: true for dump output string assertions
• Make DriveInfo middleware expectations derive the platform-specific drive name
• Keeps production output unchanged while allowing tests to pass on Linux and Windows line endings

Verification

• dotnet build VarDump.sln --configuration Release --no-restore
• dotnet test test/VarDump.UnitTests/VarDump.UnitTests.csproj --configuration Release --no-build --framework net6.0
• dotnet test test/VarDump.UnitTests/VarDump.UnitTests.csproj --configuration Release --no-build --framework net9.0
• dotnet test test/VarDump.Extensions.UnitTests/VarDump.Extensions.UnitTests.csproj --configuration Release --no-build --framework net6.0
• dotnet test test/VarDump.Extensions.UnitTests/VarDump.Extensions.UnitTests.csproj --configuration Release --no-build --framework net9.0