This makes it easier to debug a frozen test or a test that failed. To
debug a failed test, remove the line fs::remove_all(m_path_root);.

The pull is done in three commits:

 - Create a datadir for every unit test once (and only once). This
requires the SetDataDir function to go away.
 - Use the common setup in the gui unit tests. Some of those tests are
testing the init sequence, so we'd have to undo some of what the testing
setup did.
 - Log to the debug.log in all tests

ninja check