lit: remove python2-isms

Summary:
`assert.assertItemEqual` went away in Python 3. Seeing how lists
are ordered, comparing a list against each other should work just
as well.

Patch by @jbergstroem (Johan Bergström).

Reviewers: modocache, gparker42

Reviewed By: modocache

Differential Revision: https://reviews.llvm.org/D31229

llvm-svn: 298479