testing/_utils/site.py: Fix missing BZR assignment

Tom Pollardrequested to merge
tpollard/bzrfix into master

Description

On a fresh install of master, the following import error is thrown when attempting to run the tests if bazaar is not installed.

tests/conftest.py:31: in <module>
    from tests.testutils.repo.bzr import Bzr
tests/testutils/repo/bzr.py:6: in <module>
    from buildstream.testing._utils.site import BZR, BZR_ENV, HAVE_BZR
E   ImportError: cannot import name 'BZR' from 'buildstream.testing._utils.site'

Ensure BZR is set to None when not available on host, in the same way we handle missing GIT.

Edited by Tom Pollard

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