patch 8.2.1752: GTK GUI: cannot map alt-? with <A-?>

Problem:    GTK GUI: cannot map alt-? with <A-?>. (Ingo Karkat)
Solution:   Adjust the characters for which the shift modifier is removed.
            (closes #7016)  Make Motif and Win32 use the same function as GTK.
diff --git a/src/testdir/test_termcodes.vim b/src/testdir/test_termcodes.vim
index f3a7af5..91ccda8 100644
--- a/src/testdir/test_termcodes.vim
+++ b/src/testdir/test_termcodes.vim
@@ -2123,6 +2123,20 @@
   call RunTest_mapping_works_with_mods(function('GetEscCodeCSIu'), 'S-A', 4)
 endfunc
 
+func Test_mapping_works_with_alt_and_shift()
+  new
+  set timeoutlen=10
+
+  " mapping <A-?> works even though the code is A-S-?
+  for c in ['!', '$', '+', ':', '?', '^', '~']
+    call RunTest_mapping_mods('<A-' .. c .. '>', c, function('GetEscCodeCSI27'), 4)
+    call RunTest_mapping_mods('<A-' .. c .. '>', c, function('GetEscCodeCSIu'), 4)
+  endfor
+
+  bwipe!
+  set timeoutlen&
+endfunc
+
 func Test_mapping_works_with_ctrl_alt()
   call RunTest_mapping_works_with_mods(function('GetEscCodeCSI27'), 'C-A', 7)
   call RunTest_mapping_works_with_mods(function('GetEscCodeCSIu'), 'C-A', 7)