autotest: Add context manager wrapper

Add a decorator that grabs a named context manager from self before
calling the wrapped function.

BUG=None
TEST=Included unit test

Change-Id: Id8271c36c98f393a6d0617f0eeb881217f5b3a8d
Reviewed-on: https://gerrit.chromium.org/gerrit/45114
Commit-Queue: Christopher Wiley <wiley@chromium.org>
Reviewed-by: Christopher Wiley <wiley@chromium.org>
Tested-by: Christopher Wiley <wiley@chromium.org>
2 files changed