Add a dummy test to NonEmptySetTest.Guava

Add a dummy test so that when IntelliJ (incorrectly) scans NonEmptySetTest.Guava
looking for tests, it finds one and doesn't complain.

core/src/test/kotlin/com/r3corda/core/utilities/NonEmptySetTest.kt

@@ -44,6 +44,13 @@ class NonEmptySetTest {
                     .suppressing(CollectionRetainAllTester::class.java.getMethods().toList())
                     .createTestSuite()
         }
+
+        /**
+         * For some reason IntelliJ really wants to scan this class for tests and fail when
+         * it doesn't find any. This stops that error from occurring.
+         */
+        @Test fun dummy() {
+        }
     }
 
     /**