test: Add build system support and a main function for unit tests

Add a new CMake option BUILD_TESTING (defaults to OFF) that, if
enabled, will build unit tests and related requirements.

Add a new library Quassel::Test::Main that provides a main function
for test cases.

Add a new CMake macro quassel_add_test for making the adding of test
cases convenient by hiding most of the boilerplate.

Test cases should #include "testglobal.h", so they automatically have
access to GTest/GMock macros as well as the imported main function.