Update NuGet dependency to 6.3.1

src/Microsoft.CodeAnalysis.Testing/Microsoft.CodeAnalysis.Analyzer.Testing/Microsoft.CodeAnalysis.Analyzer.Testing.csproj

@@ -31,7 +31,7 @@
     </When>
     <Otherwise>
       <PropertyGroup>
-        <NuGetApiVersion>6.3.0</NuGetApiVersion>
+        <NuGetApiVersion>6.3.1</NuGetApiVersion>
       </PropertyGroup>
     </Otherwise>
   </Choose>