This allows us to compile the engine tests against the internal VAPI, so
we can unit test internal classes.

* CMakeLists.txt: Add top-level targets test-engine-run and
  test-client-run that execute the respective test binaries. Make
  the tests target depend on them and make them depend on their
  binaries.

* Makefile.in: Add test-client and test-engin targets for convenience.

* src/CMakeLists.txt: Make the right invocation of valac to generate a
  correct geary-engine-internal.vapi. Make gsettings schema generation
  depend on geary-client so it exists for test-client if the main binary
  hasn't been built.

* test/CMakeLists.txt: Split everything up into to builds, one for
  test-engine and the other for test-client. Use geary-engine-internal
  when building test-engine for access to internal classes and members.

* test/engine/util-idle-manager-test.vala,
  test/engine/util-timeout-manager-test.vala: Use Glib MainContext rather
  than the GTK main loop for pumping events so the test cases compile
  without GTK.

* test/test-engine.vala, test/test-client.vala: New main source file for
  test binaries based on old main.vala.