patch 8.1.1193: typos and small problems in test files

Problem:    Typos and small problems in test files.
Solution:   Small improvements.
diff --git a/src/testdir/test_gn.vim b/src/testdir/test_gn.vim
index d0fdafc..fc7e891 100644
--- a/src/testdir/test_gn.vim
+++ b/src/testdir/test_gn.vim
@@ -2,7 +2,7 @@
 
 func Test_gn_command()
   noautocmd new
-  " replace a single char by itsself quoted:
+  " replace a single char by itself quoted:
   call setline('.', 'abc x def x ghi x jkl')
   let @/ = 'x'
   exe "norm! cgn'x'\<esc>.."