Separate wait_until to standalone function

It makes wait_until more usable outside of unittest.TestCase classes.
Part of this patch is renaming pinger module to helpers thus we can have
helpers module with useful utilities for testing.

Change-Id: Ie8e4854c23aa3739b0df618db1b3944466d64bd2
Related-Bug: 1243216
Merge "Separate wait_until to standalone function"