* test/client/application/geary-configuration-test.vala: New unit test
for Configuration class.
* test/main.vala: Add new unit test to the client suite, ensure the
memory GSettings backend is used as the default so we get default
setting values, and never save any changes made.
* test/CMakeLists.txt: Add new unit test source.
* src/client/application/geary-config.vala: Tidy up code a bit to adhere
to code conventions.
