patch 8.2.3016: confusing error when expression is followed by comma

Problem:    Confusing error when expression is followed by comma.
Solution:   Give a different error for trailing text. (closes #8395)
diff --git a/src/testdir/test_let.vim b/src/testdir/test_let.vim
index 1b8f74b..c05a4cb 100644
--- a/src/testdir/test_let.vim
+++ b/src/testdir/test_let.vim
@@ -314,6 +314,7 @@
     let ch = test_null_channel()
     call assert_fails('let ch += 1', 'E734:')
   endif
+  call assert_fails('let name = "a" .. "b",', 'E488: Trailing characters: ,')
 
   " This test works only when the language is English
   if v:lang == "C" || v:lang =~ '^[Ee]n'