tools: move tree-diff into ./tools

Move the `tree-diff` tool into ./tools, which is our new place for tools
used by the test-suite or during development.

The only hard-coded user is the TestBase, so fix its path to the tool
so the test-suite will continue to find it.

test/test.py

@@ -156,7 +156,7 @@ class TestBase():
         """
 
         checkout = TestBase.locate_test_checkout()
-        output = subprocess.check_output([os.path.join(checkout, "tree-diff"), path1, path2])
+        output = subprocess.check_output([os.path.join(checkout, "tools/tree-diff"), path1, path2])
         return json.loads(output)