build script: update to xunit 2.4.0

We've updated to xunit 2.4.0; update the build script to match.

Author: ethomson

buildandtest.cmd

@@ -31,7 +31,7 @@ dotnet build "%~dp0\" /v:minimal /nologo /property:ExtraDefine="%EXTRADEFINE%"
 @IF ERRORLEVEL 1 EXIT /B %ERRORLEVEL%
 
 :: Run tests on Desktop and CoreCLR
-"%userprofile%\.nuget\packages\xunit.runner.console\2.3.1\tools\net452\xunit.console.exe" "%~dp0bin\LibGit2Sharp.Tests\%Configuration%\net461\LibGit2Sharp.Tests.dll" -noshadow
+"%userprofile%\.nuget\packages\xunit.runner.console\2.4.0\tools\net452\xunit.console.exe" "%~dp0bin\LibGit2Sharp.Tests\%Configuration%\net461\LibGit2Sharp.Tests.dll" -noshadow
 @IF ERRORLEVEL 1 EXIT /B %ERRORLEVEL%
 dotnet test "%~dp0LibGit2Sharp.Tests/LibGit2Sharp.Tests.csproj" -f netcoreapp2.0 --no-restore --no-build
 @IF ERRORLEVEL 1 EXIT /B %ERRORLEVEL%