patch 8.2.3792: setting *func options insufficiently tested

Problem:    Setting *func options insufficiently tested.
Solution:   Impove tests. (Yegappan Lakshmanan, closes #9337)
diff --git a/src/testdir/test_quickfix.vim b/src/testdir/test_quickfix.vim
index c8e04d8..283e943 100644
--- a/src/testdir/test_quickfix.vim
+++ b/src/testdir/test_quickfix.vim
@@ -5287,6 +5287,13 @@
   let lines =<< trim END
     set efm=%f:%l:%c:%m
 
+    #" Test for using a function name
+    LET &qftf = 'g:Tqfexpr'
+    cexpr "F0:0:0:L0"
+    copen
+    call assert_equal('F0-L0C0-L0', getline(1))
+    cclose
+
     #" Test for using a function()
     set qftf=function('g:Tqfexpr')
     cexpr "F1:1:1:L1"