patch 8.2.2697: function list test fails

Problem:    Function list test fails.
Solution:   Add missing function. (Yegappan Lakshmanan)

runtime/doc/usr_41.txt

@@ -1003,6 +1003,7 @@ Testing:				    *test-functions*
 	assert_true()		assert that an expression is true
 	assert_exception()	assert that a command throws an exception
 	assert_beeps()		assert that a command beeps
+	assert_nobeep()		assert that a command does not cause a beep
 	assert_fails()		assert that a command fails
 	assert_report()		report a test failure
 	test_alloc_fail()	make memory allocation fail