patch 8.2.0535: regexp patterns not fully tested

Problem:    Regexp patterns not fully tested.
Solution:   Add more regexp tests and others. (Yegappan Lakshmanan,
            closes #5901)
diff --git a/src/testdir/test_options.vim b/src/testdir/test_options.vim
index ef3ae1a..e1b11e9 100644
--- a/src/testdir/test_options.vim
+++ b/src/testdir/test_options.vim
@@ -677,6 +677,15 @@
   bwipe!
 endfunc
 
+" Test for the 'shell' option
+func Test_shell()
+  CheckUnix
+  let save_shell = &shell
+  set shell=
+  call assert_fails('shell', 'E91:')
+  let &shell = save_shell
+endfunc
+
 " Test for the 'shellquote' option
 func Test_shellquote()
   CheckUnix