patch 8.2.1563: Vim9: error when using '%" with setbufvar() r getbufvar()

Problem:    Vim9: error when using '%" with setbufvar() or getbufvar().
Solution:   Use tv_get_buf_from_arg(). (closes #6816)

src/testdir/test_vim9_func.vim

@@ -1426,6 +1426,9 @@ def Test_setbufvar()
    settabwinvar(1, 1, '&ts', 15)
    assert_equal(15, &ts)
    setlocal ts=8
+
+   setbufvar('%', 'myvar', 123)
+   assert_equal(123, getbufvar('%', 'myvar'))
 enddef
 
 def Test_setreg()