Add tests (based on libgee tests).

 * Add tests for following methods:
     - comp_version
     - split_path (already fix some issues)
 * Use libgee's testcase.vala for some GLib.Tests abstractions.

 * CMake integration not completed because some files are required both for
   tests and also Valama executable itself. And some source files currently
   are compiled twice. This leads to some unnecessary recompile issues.

 * NOTE: Build system test features aren't used at all to keep tests
         independent of build system (but functionality is limited therefore).
         E.g. tests will completely fail on a single failure and it's not
         possible to disable tests without modifying source code (it's easy
         though).