patch 8.1.1826: tests use hand coded feature and option checks

Problem:    Tests use hand coded feature and option checks.
Solution:   Use the commands from check.vim in more tests.
diff --git a/src/testdir/test_filechanged.vim b/src/testdir/test_filechanged.vim
index a937bb8..bfcdb76 100644
--- a/src/testdir/test_filechanged.vim
+++ b/src/testdir/test_filechanged.vim
@@ -1,9 +1,10 @@
 " Tests for when a file was changed outside of Vim.
 
+source check.vim
+
 func Test_FileChangedShell_reload()
-  if !has('unix')
-    return
-  endif
+  CheckUnix
+
   augroup testreload
     au FileChangedShell Xchanged_r let g:reason = v:fcs_reason | let v:fcs_choice = 'reload'
   augroup END
@@ -91,9 +92,8 @@
 endfunc
 
 func Test_file_changed_dialog()
-  if !has('unix') || has('gui_running')
-    return
-  endif
+  CheckUnix
+  CheckNotGui
   au! FileChangedShell
 
   new Xchanged_d