(build) Fix Test Coverage

Not sure why this broke as a result of the recent changes made by me, but
it did.  Excluding the NUnit3 assembly from Test Coverage fixes the
problem.

recipe.cake

@@ -20,7 +20,7 @@ BuildParameters.PrintParameters(Context);
 ToolSettings.SetToolSettings(
     context: Context,
     dupFinderExcludePattern: new string[] { BuildParameters.RootDirectoryPath + "/src/Cake.AppCenter.Tests/*.cs" },
-    testCoverageFilter: "+[*]* -[nunit.*]* -[Cake.Core]* -[Cake.Testing]* -[*.Tests]* ",
+    testCoverageFilter: "+[*]* -[nunit.*]* -[Cake.Core]* -[Cake.Testing]* -[*.Tests]* -[NUnit3.*]*",
     testCoverageExcludeByAttribute: "*.ExcludeFromCodeCoverage*",
     testCoverageExcludeByFile: "*/*Designer.cs;*/*.g.cs;*/*.g.i.cs");
 Build.RunDotNetCore();