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_options.vim b/src/testdir/test_options.vim
index fa75204..cd66cdf 100644
--- a/src/testdir/test_options.vim
+++ b/src/testdir/test_options.vim
@@ -486,7 +486,7 @@
   " but don't exhaustively validate their results.
   call assert_equal('single', getcompletion('set ambw=', 'cmdline')[0])
   call assert_match('light\|dark', getcompletion('set bg=', 'cmdline')[1])
-  call assert_equal('indent', getcompletion('set backspace=', 'cmdline')[0])
+  call assert_equal('indent,eol,start', getcompletion('set backspace=', 'cmdline')[0])
   call assert_equal('yes', getcompletion('set backupcopy=', 'cmdline')[1])
   call assert_equal('backspace', getcompletion('set belloff=', 'cmdline')[1])
   call assert_equal('min:', getcompletion('set briopt=', 'cmdline')[1])