Update to NUnit 4.3.1

Author: manfred-brands

src/nunit.analyzers.tests/nunit.analyzers.tests.csproj

@@ -22,7 +22,7 @@
   </ItemGroup>
 
   <ItemGroup Condition="'$(NUnitVersion)' == '4'">
-    <PackageReference Include="NUnit" Version="4.2.2" />
+    <PackageReference Include="NUnit" Version="4.3.1" />
   </ItemGroup>
 
   <ItemGroup Condition="'$(NUnitVersion)' == '3'">