test/py: Use range() rather than xrange()

In python 3.x the xrange() function has been removed, and range()
returns an iterator much like Python 2.x's xrange(). Simply use range()
in place of xrange() in order to work on both python 2.x & 3.x. This
will mean a small cost on python 2.x since range() will return a list
there rather than an iterator, but the cost should be negligible.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>