Test levels reachability

 #2132, being work in progress, temporarily disabled access to level *village-forest-2* and beyond. This had no effect on tests.

The new test checks reachability of levels in two parts.
The first part checks all levels in the *maps* directory and prints warning for non-accessible levels. 
There are currently 3 levels that are not reachable via doors: *mountain_pass*, *oriental-mountains*, *oriental-temple* (presumably work in progress).

The second part checks few hand-picked levels and fails the test if the level is not reachable.
(mostly towns and levels with cut-scenes): *studyroom*, *forest*, *town*, *valley-tacotown*, *valley-3*, *rainbow-bar*, *seabluff*, *castle-hawkthorne-throne*, *greendale-exterior*.