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_autocmd.vim b/src/testdir/test_autocmd.vim
index 531aa6b..d6f8ef4 100644
--- a/src/testdir/test_autocmd.vim
+++ b/src/testdir/test_autocmd.vim
@@ -1279,8 +1279,8 @@
   call assert_equal(g:opt[0], g:opt[1])
 
   " 14: Setting option backspace through :let"
-  let g:options = [['backspace', '', '', '', 'eol,indent,start', 'global', 'set']]
-  let &bs = "eol,indent,start"
+  let g:options = [['backspace', 'indent,eol,start', 'indent,eol,start', 'indent,eol,start', '', 'global', 'set']]
+  let &bs = ''
   call assert_equal([], g:options)
   call assert_equal(g:opt[0], g:opt[1])