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_debugger.vim b/src/testdir/test_debugger.vim
index 0c4bc87..c05f4e1 100644
--- a/src/testdir/test_debugger.vim
+++ b/src/testdir/test_debugger.vim
@@ -360,16 +360,16 @@
   writefile(lines, 'Xtest.vim')
 
   # Start Vim in a terminal
-  var buf = RunVimInTerminal('-S Xtest.vim', {wait_for_ruler: 0})
-  call TermWait(buf)
+  var buf = g:RunVimInTerminal('-S Xtest.vim', {wait_for_ruler: 0})
+  call g:TermWait(buf)
 
   # Despite the failure the functions are defined
-  RunDbgCmd(buf, ':function g:EarlyFunc',
+  g:RunDbgCmd(buf, ':function g:EarlyFunc',
      ['function EarlyFunc()', 'endfunction'], {match: 'pattern'})
-  RunDbgCmd(buf, ':function g:LaterFunc',
+  g:RunDbgCmd(buf, ':function g:LaterFunc',
      ['function LaterFunc()', 'endfunction'], {match: 'pattern'})
 
-  call StopVimInTerminal(buf)
+  call g:StopVimInTerminal(buf)
   call delete('Xtest.vim')
 enddef
 
@@ -386,13 +386,13 @@
   writefile(lines, 'Xtest.vim')
 
   # Start Vim in a terminal
-  var buf = RunVimInTerminal('-S Xtest.vim', {wait_for_ruler: 0})
-  call TermWait(buf)
+  var buf = g:RunVimInTerminal('-S Xtest.vim', {wait_for_ruler: 0})
+  call g:TermWait(buf)
 
-  RunDbgCmd(buf, ':call GetNum()',
+  g:RunDbgCmd(buf, ':call GetNum()',
      ['line 1: return 1  + 2  + 3'], {match: 'pattern'})
 
-  call StopVimInTerminal(buf)
+  call g:StopVimInTerminal(buf)
   call delete('Xtest.vim')
 enddef