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_syntax.vim

@@ -84,9 +84,7 @@ func Test_syntax_after_reload()
 endfunc
 
 func Test_syntime()
-  if !has('profile')
-    return
-  endif
+  CheckFeature profile
 
   syntax on
   syntime on
@@ -119,9 +117,7 @@ func Test_syntime()
 endfunc
 
 func Test_syntime_completion()
-  if !has('profile')
-    return
-  endif
+  CheckFeature profile
 
   call feedkeys(":syntime \<C-A>\<C-B>\"\<CR>", 'tx')
   call assert_equal('"syntime clear off on report', @:)
@@ -498,9 +494,8 @@ func Test_bg_detection()
 endfunc
 
 func Test_syntax_hangs()
-  if !has('reltime') || !has('float') || !has('syntax')
-    return
-  endif
+  CheckFunction reltimefloat
+  CheckFeature syntax
 
   " This pattern takes a long time to match, it should timeout.
   new
@@ -532,9 +527,7 @@ func Test_syntax_hangs()
 endfunc
 
 func Test_conceal()
-  if !has('conceal')
-    return
-  endif
+  CheckFeature conceal
 
   new
   call setline(1, ['', '123456'])