patch 8.2.1576: Vim9: index() does not take "true" as argument

Problem:    Vim9: index() does not take "true" as argument.
Solution:   Use tv_get_bool_chk(). (closes #6823)
diff --git a/src/testdir/test_vim9_func.vim b/src/testdir/test_vim9_func.vim
index 11a6ddd..04f8d7a 100644
--- a/src/testdir/test_vim9_func.vim
+++ b/src/testdir/test_vim9_func.vim
@@ -1497,6 +1497,10 @@
   assert_equal(0, count('ABC ABC ABC', 'b', false))
 enddef
 
+def Test_index()
+  assert_equal(3, index(['a', 'b', 'a', 'B'], 'b', 2, true))
+enddef
+
 def Test_expand()
   split SomeFile
   assert_equal(['SomeFile'], expand('%', true, true))