patch 8.2.1571: Vim9: count() third argument cannot be "true"

Problem:    Vim9: count() third argument cannot be "true".
Solution:   use tv_get_bool_chk(). (closes #6818)
diff --git a/src/testdir/test_vim9_func.vim b/src/testdir/test_vim9_func.vim
index 480a9a5..8f4928a 100644
--- a/src/testdir/test_vim9_func.vim
+++ b/src/testdir/test_vim9_func.vim
@@ -1492,6 +1492,11 @@
   endif
 enddef
 
+def Test_count()
+  assert_equal(3, count('ABC ABC ABC', 'b', true))
+  assert_equal(0, count('ABC ABC ABC', 'b', false))
+enddef
+
 def Test_recursive_call()
   assert_equal(6765, Fibonacci(20))
 enddef