Replace assert_ with assertTrue

The method assert_ has been deprecated since python 2.7.
http://docs.python.org/2/library/unittest.html#deprecated-aliases

This patch is a mere sed replace of one-liner assert_ to assertTrue.

grep -lR "assert_(" tests/* | \
xargs sed -ri 's/(\W+self.assert)_(\()(.+)(\)$)/\1True\2\3\4/'

Change-Id: Ic817bf32704adf2eb6ebbe67169625d65d49a9ad
Merge "Replace assert_ with assertTrue"