Switch to NUnit for V4/V5 tests (get rid of MS Tests and XUnit). Code cleanup in tests. Actualize tests.

Author: Dmitry Koval

route4me-csharp-sdk/Route4MeSDKUnitTest/Route4MeManagerUnitTest.cs

@@ -212,8 +212,7 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.0.0" />
-    <PackageReference Include="MSTest.TestAdapter" Version="2.2.8" />
-    <PackageReference Include="MSTest.TestFramework" Version="2.2.8" />
+    <PackageReference Include="NUnit" Version="3.13.3" />
     <PackageReference Include="System.CodeDom" Version="6.0.0" />
   </ItemGroup>