patch 8.2.1590: Vim9: bufnr() doesn't take "true" argument

Problem:    Vim9: bufnr() doesn't take "true" argument.
Solution:   use tv_get_bool_chk(). (closes #6863)

src/testdir/test_vim9_func.vim

@@ -1327,6 +1327,10 @@ enddef
 def Test_bufnr()
   let buf = bufnr()
   assert_equal(buf, bufnr('%'))
+
+  buf = bufnr('Xdummy', true)
+  assert_notequal(-1, buf)
+  exe 'bwipe! ' .. buf
 enddef
 
 def Test_col()