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_sort.vim

@@ -3,12 +3,12 @@
 source check.vim
 
 func Compare1(a, b) abort
-    call sort(range(3), 'Compare2')
-    return a:a - a:b
+  call sort(range(3), 'Compare2')
+  return a:a - a:b
 endfunc
 
 func Compare2(a, b) abort
-    return a:a - a:b
+  return a:a - a:b
 endfunc
 
 func Test_sort_strings()