fix: use correct base path in Directory.CreateTempSubdirectory (#1127)

* Add failing test
* Use correct path for temporary directory

Author: vbreuss

src/TestableIO.System.IO.Abstractions.TestingHelpers/MockDirectory.cs

@@ -118,8 +118,8 @@ public override IDirectoryInfo CreateTempSubdirectory(string prefix = null)
             // Perform directory name generation in a loop, just in case the randomly generated name already exists.
             do
             {
-                var randomDir = $"{prefix}{Path.GetRandomFileName()}";
-                potentialTempDirectory = Path.Combine(Path.GetTempPath(), randomDir);
+                var randomDir = $"{prefix}{FileSystem.Path.GetRandomFileName()}";
+                potentialTempDirectory = Path.Combine(FileSystem.Path.GetTempPath(), randomDir);
             } while (Exists(potentialTempDirectory));
 
             return CreateDirectoryInternal(potentialTempDirectory);

tests/TestableIO.System.IO.Abstractions.TestingHelpers.Tests/MockDirectoryTests.cs

@@ -862,7 +862,7 @@ public void MockDirectory_CreateTempSubdirectory_ShouldCreateSubdirectoryInTempD
 
             // Assert
             Assert.That(fileSystem.Directory.Exists(result.FullName), Is.True);
-            Assert.That(result.FullName.Contains(Path.GetTempPath()), Is.True);
+            Assert.That(result.FullName, Does.StartWith(fileSystem.Path.GetTempPath()));
         }
 
         [Test]
@@ -877,7 +877,7 @@ public void MockDirectory_CreateTempSubdirectoryWithPrefix_ShouldCreateDirectory
             // Assert
             Assert.That(fileSystem.Directory.Exists(result.FullName), Is.True);
             Assert.That(Path.GetFileName(result.FullName).StartsWith("foo-"), Is.True);
-            Assert.That(result.FullName.Contains(Path.GetTempPath()), Is.True);
+            Assert.That(result.FullName.Contains(fileSystem.Path.GetTempPath()), Is.True);
         }
 #endif