patch 9.1.0899: default for 'backspace' can be set in C code

Problem:  default for 'backspace' can be set in C code
Solution: promote the default for 'backspace' from defaults.vim to the C
          code (Luca Saccarola)

closes: #16143

Signed-off-by: Luca Saccarola <github.e41mv@aleeas.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_digraph.vim b/src/testdir/test_digraph.vim
index 3312faf..16eb0b1 100644
--- a/src/testdir/test_digraph.vim
+++ b/src/testdir/test_digraph.vim
@@ -250,9 +250,12 @@
   call Put_Dig_BS("P","=")
   call assert_equal(['Р']+repeat(["₽"],2)+['П'], getline(line('.')-3,line('.')))
   " Not a digraph: this is different from <c-k>!
+  let _bs = &bs
+  set bs=
   call Put_Dig_BS("a","\<bs>")
   call Put_Dig_BS("\<bs>","a")
   call assert_equal(['','a'], getline(line('.')-1,line('.')))
+  let &bs = _bs
   " Grave
   call Put_Dig_BS("a","!")
   call Put_Dig_BS("!","e")