python tests: assert string equality, with diff

python tests: assert string equality, with diff

In the success case this works just like self.assertEqual(),
but when things fail you get a better representation of where it went
wrong (a unified diff).

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>