mark test_navigation_buttons as flaky

(cherry-picked from 292b425c)

common/test/acceptance/tests/lms/test_lms_courseware.py

@@ -436,6 +436,7 @@ class CoursewareMultipleVerticalsTest(UniqueCourseTest, EventsTestMixin):
         self.courseware_page.visit()
         self.course_nav = CourseNavPage(self.browser)
 
+    @flaky #TODO: fix this, see TNL-5762
     def test_navigation_buttons(self):
         # start in first section
         self.assert_navigation_state('Test Section 1', 'Test Subsection 1,1', 0, next_enabled=True, prev_enabled=False)