test_service.py: Fix PEP8 since 7562b25

For some reason this code was failing PEP8 on my machine, while it was
passing on Dave's.  This change makes it pass on both.
test_service.py: Fix test-failure due to rebasing error

7562b25 introduces a new test `test_that_get_ref_gets_ref` which
references the heads in the test-pack bundle.  This bundle was changed in
b055d97 and it looks like 7562b25 was not updated to reflect that fact
after a rebase.
run: Rename "test-pack" to "test-pack-checkout-cache"

Dave reported that the test-pack he was testing with was being deleted
when running a test.  This is because it was named `test-pack` which
stbt-service expects to use to cache a checkout of the test-pack so it
doesn't need to do a fresh checkout from bare every-time.  If it finds
the cache is invalid it will blow it away, leading to the symptom Dave
saw.

I'm certain that this has caused us problems before so the fix is to
rename the directory to something more explicit and less likely to be
tripped-over.