The `Bridge` code was written before we had good mocking tools in the
client. As a result the `Bridge` class mocks `RPC` in an unconventional
way where it constructs the real class, but then stubs out specific
methods. Also tests varied in the degree to which they tested only
`Bridge` vs Bridge + RPC together.

This commit mocks `RPC` in bridge-test.js using our standard mocking
tools and makes all tests in the file unit tests. This involved some
significant changes to the `onConnect` tests.