patch 9.0.1227: no cmdline completion for :runtime

Problem:    No cmdline completion for :runtime.
Solution:   Add completion for :runtime. (closes #11853, closes #11447)
            Improve the resulting matches.
diff --git a/src/testdir/test_cmdline.vim b/src/testdir/test_cmdline.vim
index 9eca918..9406a7e 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -552,6 +552,15 @@
   call assert_true(index(l, '<buffer>') >= 0)
   let l = getcompletion('not', 'mapclear')
   call assert_equal([], l)
+  
+  let l = getcompletion('', 'runtime')
+  call assert_true(index(l, 'defaults.vim') >= 0)
+  let l = getcompletion('synt', 'runtime')
+  call assert_true(index(l, 'syntax') >= 0)
+  let l = getcompletion('syntax/vi', 'runtime')
+  call assert_true(index(l, 'syntax/vim.vim') >= 0)
+  let l = getcompletion('notexitsts', 'runtime')
+  call assert_equal([], l)
 
   let l = getcompletion('.', 'shellcmd')
   call assert_equal(['./', '../'], filter(l, 'v:val =~ "\\./"'))