This change formalizes part of the setup process of SCM unit tests. Most

tests need to set up a checkout or clone of a repository that they can

run tests against, and as tests often need to modify the repository,

these checkouts needed to be freshly re-made on each test run. This was

really slow.
Now, SCMClientTestCase (the new name for SCMClientTests) provides

central management of checkouts that supports caching of checkouts. Upon

suite setup, a directory will be created where checkouts can go. There's

a working area, which the subclass can populate with as many or as few

checkouts as the suite needs, and there's a cache.
Subclasses that implement setup_checkout() will place these checkouts

in the working area path. SCMClientTestCase then places that in the

cache, and deletes it when the suite tears down.
On each test run, the cached copy is restored back to the workarea

through a tree copy, which is generally faster than a clone/checkout

operation. The tests can do what they need with the checkout. That

checkout will then be cleared away and repopulated on the next test run.
In my tests here, this effectively reduces the rbtools.clients.tests

time by about 30-40 seconds.

Unit tests pass.