Switch to use test_utils.call_until_true

test.call_until_true has been deprecated since Newton on Tempest side,
and now Tempest provides test_utils.call_until_true as the stable
library method. So this patch switches to use the stable method before
removing old test.call_until_true on Tempest side.

Change-Id: I5e82466f6eb7b164042406813b1ad72a1cbb05fa
1 file changed