Don't statically cache InUnitTestRunner.

This was causing random test failures.

Fixes #1136

Author: grokys

src/GitHub.Extensions/Guard.cs

@@ -60,6 +60,6 @@ public static void ArgumentInRange(int value, int minValue, int maxValue, string
 
         // Borrowed from Splat.
 
-        public static bool InUnitTestRunner { get; } = Splat.ModeDetector.InUnitTestRunner();
+        public static bool InUnitTestRunner => Splat.ModeDetector.InUnitTestRunner();
     }
 }