Tweak import to simplify dependency handling.

At the moment, the sqlitewrappers also import the mailer, meaning
that anyone wanting to import just one of the wrappers needs to either
import the whole module, or import something else which does so first.
This is a non-ideal fix, but provides some improvement.

Change-Id: I1b61c346ca24cc9dc469bd1dd8a8d0aa3e1d006a
Break out some common test helpers to their own file.

Change-Id: If226f45c4464b768d655338be724f4c5e3a59372
Extract unit tests from the api tests, add test runner script.

This creates a new unit-tests directory under test/, which is used
to contain tests which test the nemesis internals directly. At some
point it may make sense for these internals and their tests to move
to libnemesis, but for now it's at least clearer what's what.

This also adds a ./run-tests script which will run all the current
tests (ie, not the client-tests) and return a summed return code.

Change-Id: I379e9e3ac7b9ff1277e540490de431d96064badb