patch 9.0.0346: :horizontal modifier not fully supported

Problem:    :horizontal modifier not fully supported.
Solution:   Also use :horizontal for completion and user commands.
            (closes #11025)
diff --git a/src/testdir/test_usercommands.vim b/src/testdir/test_usercommands.vim
index 9c75a91..91d8bfd 100644
--- a/src/testdir/test_usercommands.vim
+++ b/src/testdir/test_usercommands.vim
@@ -103,6 +103,10 @@
   call assert_equal('vertical', g:mods)
   vert MyCmd
   call assert_equal('vertical', g:mods)
+  horizontal MyCmd
+  call assert_equal('horizontal', g:mods)
+  hor MyCmd
+  call assert_equal('horizontal', g:mods)
 
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