Improved AStarTest::tearDown() which now also deletes board instance.
Enhanced AStarTest::testGetPath().

This now checks if the correct steps were added to the path.
Added a unit which can be used as blocker to AStarTest.

This unit can be set on a square which shall not be accessible for the unit. It's kind of a "walL".
Implemented an extended test for qrw::AStar::findPath().