Add png dumping to LOK tiled rendering test.

This allows for easier visual comparisons (i.e. currently the test
would be failing for some tiles).

Change-Id: I5b174375b57ffe0edd2700fdec411a83669e4a34
libreofficekit_tiledrendering test: force lock file removal

If the lock file still exists when running this test, LOK will fail
resulting in "documentLoad failed: unknown load failure"
(the actual error is that the lock file dialog cannot be confirmed
 by the user in headless mode, resulting in loading failure, however
 this is then hidden by multiple layers of exception redirection
 in sfx2).

Change-Id: I025ea6187c3d17805f25ab6f756eae9646f2c7c8