Work towards fixing the "do-not-update-to-rcs" self-test.

The test was failing because it was written to run against the real
local warehouse, instead of a stubbed one. When initializing the
Sandbox with an explicit warehouse, the test failed because of
incorrect logic piping the right path to the packages database
(in this case "test-packages.data.db" instead of "packages.data.db").

Unfortunately, the test still fails -- this time potentially due to a bug:
After publishing the package for the first time with `--create`,
the next step tries to update but gets the following error:
    There is no package named test:i1iyhj. If you are creating a
    new package, use the --create flag.