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_diffmode.vim

@@ -1,4 +1,5 @@
 " Tests for diff mode
+
 source shared.vim
 source screendump.vim
 source check.vim
@@ -654,9 +655,8 @@ endfunc
 
 func Test_diffpatch()
   " The patch program on MS-Windows may fail or hang.
-  if !executable('patch') || !has('unix')
-    return
-  endif
+  CheckExecutable patch
+  CheckUnix
   new
   insert
 ***************