Added explicit wait for payment page to load

Fix for this failure observed in master: https://jenkins.testeng.edx.org/job/edx-all-tests-auto-master/376/SHARD=2,TEST_SUITE=lms-acceptance/

The screenshot verifies that the page loaded correctly; the assertion just executed too soon.
The fix adds an explicit wait (with a timeout) for the element to appear.

@dianakhuang