Add a reference to the OpenApi.Tests project to access the string extension method

Author: MaggieKimani1

test/Microsoft.OpenApi.Readers.Tests/Microsoft.OpenApi.Readers.Tests.csproj

@@ -28,6 +28,7 @@
     <ItemGroup>
         <ProjectReference Include="..\..\src\Microsoft.OpenApi\Microsoft.OpenApi.csproj" />
         <ProjectReference Include="..\..\src\Microsoft.OpenApi.Readers\Microsoft.OpenApi.Readers.csproj" />
+        <ProjectReference Include="..\Microsoft.OpenApi.Tests\Microsoft.OpenApi.Tests.csproj" />
     </ItemGroup>
 
 </Project>