patch 8.2.1432: various inconsistencies in test files

Problem:    Various inconsistencies in test files.
Solution:   Add modelines where they were missing.  Use Check commands instead
            of silently skipping over tests.  Adjust indents and comments.
            (Ken Takata, closes #6695)

src/testdir/test_lambda.vim

@@ -1,5 +1,7 @@
 " Test for lambda and closure
 
+source check.vim
+
 func Test_lambda_feature()
   call assert_equal(1, has('lambda'))
 endfunc
@@ -19,9 +21,7 @@ func Test_lambda_with_sort()
 endfunc
 
 func Test_lambda_with_timer()
-  if !has('timers')
-    return
-  endif
+  CheckFeature timers
 
   let s:n = 0
   let s:timer_id = 0