Test case for TreeInfo#defPath

I'm about to refactor that method in terms of a TreeAccumulator.

Note that I've packaged the test case in `dotty.tools.dotc.ast`
 I believe this is the best approach for organizing unit tests:
 the import tax is minimized, and use of relative imports is
 made less fragile by avoiding creating new packages to test code.

I'll reorganize the other unit tests like this if others agree.
Refactor TreeInfo#defPath

Uses a TreeAccumulator, rather than ad-hoc descent through `productIterator`.
Merge pull request #76 from retronym/topic/defPath-accum

Test and refactor TreeInfo#defPath