patch 8.2.1799: some Normal mode commands not fully tested

Problem:    Some Normal mode commands not fully tested.
Solution:   Add a few more tests. (Yegappan Lakshmanan, closes #7073)
diff --git a/src/testdir/test_gf.vim b/src/testdir/test_gf.vim
index deede22..5e5de41 100644
--- a/src/testdir/test_gf.vim
+++ b/src/testdir/test_gf.vim
@@ -183,6 +183,21 @@
   au! InsertCharPre
 
   bwipe!
+
+  " gf is not allowed when buffer is locked
+  new
+  augroup Test_gf
+    au!
+    au OptionSet diff norm! gf
+  augroup END
+  call setline(1, ['Xfile1', 'line2', 'line3', 'line4'])
+  call test_override('starting', 1)
+  call assert_fails('diffthis', 'E788:')
+  call test_override('starting', 0)
+  augroup Test_gf
+    au!
+  augroup END
+  bw!
 endfunc
 
 " If a file is not found by 'gf', then 'includeexpr' should be used to locate