patch 8.2.2931: Vim9: line continuation comment uses legacy syntax

Problem:    Vim9: line continuation comment still uses legacy syntax in one
            place.
Solution:   Check for #\ instead of "\ earlier. (closes #8316)
diff --git a/src/testdir/test_vim9_script.vim b/src/testdir/test_vim9_script.vim
index ee32495..d5951c6 100644
--- a/src/testdir/test_vim9_script.vim
+++ b/src/testdir/test_vim9_script.vim
@@ -3081,6 +3081,9 @@
       \ start='foo'
       #\ comment
       \ end='bar'
+      syn region Text start='foo'
+      #\ comment
+      \ end='bar'
   END
   CheckScriptSuccess(lines)