patch 8.2.4257: Vim9: finding global function without g: prefix inconsistent

Problem:    Vim9: finding global function without g: prefix but not finding
            global variable is inconsistent.
Solution:   Require using g: for a global function.  Change the vim9.vim
            script into a Vim9 script with exports.  Fix that import in legacy
            script does not work.
diff --git a/src/testdir/test_usercommands.vim b/src/testdir/test_usercommands.vim
index b3ba620..331e55f 100644
--- a/src/testdir/test_usercommands.vim
+++ b/src/testdir/test_usercommands.vim
@@ -1,6 +1,6 @@
 " Tests for user defined commands
 
-source vim9.vim
+import './vim9.vim' as v9
 
 " Test for <mods> in user defined commands
 function Test_cmdmods()
@@ -287,13 +287,13 @@
       vim9script
       com! -complete=file DoCmd :
   END
-  call CheckScriptFailure(lines, 'E1208', 2)
+  call v9.CheckScriptFailure(lines, 'E1208', 2)
 
   let lines =<< trim END
       vim9script
       com! -nargs=0 -complete=file DoCmd :
   END
-  call CheckScriptFailure(lines, 'E1208', 2)
+  call v9.CheckScriptFailure(lines, 'E1208', 2)
 
   com! -nargs=0 DoCmd :
   call assert_fails('DoCmd x', 'E488:')
@@ -645,7 +645,7 @@
       command DoesNotEnd {
          echo 'hello'
   END
-  call CheckScriptFailure(lines, 'E1026:')
+  call v9.CheckScriptFailure(lines, 'E1026:')
 
   let lines =<< trim END
       command HelloThere {
@@ -653,7 +653,7 @@
         }
       HelloThere
   END
-  call CheckScriptSuccess(lines)
+  call v9.CheckScriptSuccess(lines)
   delcommand HelloThere
 
   let lines =<< trim END
@@ -664,7 +664,7 @@
           }
       BadCommand
   END
-  call CheckScriptFailure(lines, 'E1128:')
+  call v9.CheckScriptFailure(lines, 'E1128:')
 endfunc
 
 func Test_delcommand_buffer()