Fix package downgrade errors by fixing versions causing error and introducing/consolidating variables

[moljac]

Description of Change

Working on other PRs and unit tests fail

dotnet build src/Compatibility/Core/tests/Compatibility.UnitTests/Compatibility.Core.UnitTests.csproj

./src/Compatibility/Core/tests/Compatibility.UnitTests/Compatibility.Core.UnitTests.csproj : error NU1605: Warning As Error: Detected package downgrade: xunit from 2.5.1 to 2.4.2. Reference the package directly from the project to select a different version. 
./src/Compatibility/Core/tests/Compatibility.UnitTests/Compatibility.Core.UnitTests.csproj : error NU1605:  Microsoft.Maui.Compatibility.Core.UnitTests -> Microsoft.Maui.TestUtils -> xunit (>= 2.5.1) 
./src/Compatibility/Core/tests/Compatibility.UnitTests/Compatibility.Core.UnitTests.csproj : error NU1605:  Microsoft.Maui.Compatibility.Core.UnitTests -> xunit (>= 2.4.2)

1. fixed (bumped) xunit version
2. introduced msbuild variables for test dependencies (consolidation kinda)