Extend test timeout

We have a test failing in 8 seconds when the timeout was set to 10. Trying 15 in an effort to give the tests enough time.

Author: AArnott

test/Microsoft.VisualStudio.Threading.Tests/TestBase.cs

@@ -20,7 +20,7 @@ public abstract class TestBase
     /// The maximum length of time to wait for something that we expect will happen
     /// within the timeout.
     /// </summary>
-    protected static readonly TimeSpan UnexpectedTimeout = Debugger.IsAttached ? Timeout.InfiniteTimeSpan : TimeSpan.FromSeconds(10);
+    protected static readonly TimeSpan UnexpectedTimeout = Debugger.IsAttached ? Timeout.InfiniteTimeSpan : TimeSpan.FromSeconds(15);
 
     /// <summary>
     /// The maximum length of time to wait for something that we do not expect will happen