patch 8.2.2369: Vim9: functions return true/false but can't be used as bool

Problem:    Vim9: functions return true/false but can't be used as bool.
Solution:   Add ret_number_bool(). (closes #7693)
diff --git a/src/testdir/test_vim9_builtin.vim b/src/testdir/test_vim9_builtin.vim
index 61d129e..50dc2a0 100644
--- a/src/testdir/test_vim9_builtin.vim
+++ b/src/testdir/test_vim9_builtin.vim
@@ -115,6 +115,21 @@
   CheckDefExecFailure(lines, 'E1131:', 2)
 enddef
 
+def Test_append()
+  new
+  setline(1, range(3))
+  var res1: number = append(1, 'one')
+  assert_equal(0, res1)
+  var res2: bool = append(3, 'two')
+  assert_equal(false, res2)
+  assert_equal(['0', 'one', '1', 'two', '2'], getline(1, 6))
+enddef
+
+def Test_buflisted()
+  var res: bool = buflisted('asdf')
+  assert_equal(false, res)
+enddef
+
 def Test_bufname()
   split SomeFile
   bufname('%')->assert_equal('SomeFile')
@@ -199,6 +214,11 @@
   CheckDefExecAndScriptFailure(lines, 'E475:')
 enddef
 
+def Test_delete()
+  var res: bool = delete('doesnotexist')
+  assert_equal(true, res)
+enddef
+
 def Test_executable()
   assert_false(executable(""))
   assert_false(executable(test_null_string()))