Attempt at fixing tests

Tests are failing on the buildbot and it looks like it's because one
test is clobbering data for another test. The only thing these tests
share is the global ServiceProvider set, so put these test classes in
the same collection so xunit doesn't parallelize them.

Author: shana

src/UnitTests/GitHub.App/Models/RepositoryModelTests.cs

@@ -53,6 +53,7 @@ public void DifferentContentEqualsFalse(string name1, string url1, string name2,
         }
     }
 
+    [Collection("PackageServiceProvider global data tests")]
     public class PathConstructorTests : TempFileBaseClass
     {
         [Fact]

src/UnitTests/GitHub.Extensions/SimpleRepositoryModelExtensionTests.cs

@@ -7,8 +7,8 @@
 using UnitTests;
 using Xunit;
 using GitHub.Primitives;
-using System.Linq;
 
+[Collection("PackageServiceProvider global data tests")]
 public class SimpleRepositoryModelExtensionTests : TempFileBaseClass
 {
     void SetupRepository(string sha)