Update stable charms to use stable CH in test reqs

In our test-requirements.txt we often pull in charm-helpers. As stable and
master branches of charm-helpers diverge this can lead to testing failures
on the stable branch charms.

Update tests-requirements.txt to point to the stable branch of charm-helpers.

Change-Id: Id6236cd45d34f999974876c32b14d7861a0fc567