Update Apache.Arrow.Tests.csproj

Continue to support .NET 4.6.2

Author: CurtHagenlocher

csharp/test/Apache.Arrow.Tests/Apache.Arrow.Tests.csproj

@@ -16,7 +16,7 @@
   </PropertyGroup>
 
   <ItemGroup Condition="'$(TargetFramework)' == 'net462'">
-    <PackageReference Include="xunit.runner.visualstudio" Version="3.1.1">
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.8.2">
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
     </PackageReference>