Add SessionFactoryManager#isStarted()

The DatabaseSelectionRule might not start our SessionFactoryManager
but a test might won't to access the factory in a `@AfterClass` method.
This method allows to check if the SessionFactoryManager is started before
requesting a SessionFactory.

Author: DavideD

hibernate-reactive-core/src/test/java/org/hibernate/reactive/testing/SessionFactoryManager.java

@@ -35,6 +35,10 @@ public void start(Supplier<SessionFactory> supplier) {
 		}
 	}
 
+	public boolean isStarted() {
+		return sessionFactory != null;
+	}
+
 	public SessionFactory getHibernateSessionFactory() {
 		return sessionFactory;
 	}