* Add network store for testing
An alternative persistent state store has been created for use with e2e
tests. Instead of reading state from disk, it tries to load state from
a local fixture server running on port `12345` and serving state from
the path `/state.json`, and returns a blank state otherwise.
* Add e2e test fixture server
A fixture server has been added for serving background state, which the
background will read upon startup as part of restoring persisted state.
The `signature-request` e2e test has been updated to use a fixture to
bypass the registration step. The fixture used (`imported-account`) was
generated by pausing midway through that test run
