Fix data race

LP # 1386968 may be unrelated to a data race, but this needed to be fixed anyway.
Responding to review feedback.
Merge branch 'master' of https://github.com/juju/juju into 207-fix-data-race-lp-1386968

Conflicts:
	dependencies.tsv
Merge pull request #1031 from davecheney/207-fix-data-race-lp-1386968

Fix data race

LP # 1386968 may be unrelated to a data race, but this needed to be fixed anyway.

* updates `charm.v4` to pick up the api change to the `testing` sub package
* roll that change through the clients of `charm.v4/testing`