tests: Add first shot at an accessibility dump tool

tests: Add first shot at an accessibility dump tool

The tool works like this:
  ./accessibility-dump [FILE ...]
If no files are given, all files with the extension ".ui" in the current
directory are taken. For every file "test.ui", the following steps are
performed:
1) test.ui is loaded using GtkBuilder
2) The accessible for the window is loaded
3) The information of accessible is converted into a string using a
   syntax defined in this test file
4) The generated string is diffed with the file "test.txt"
5) If the diff is empty, the test is a success, if not, the test fails.
6) The diff is output when the test runner is run with --verbose

So to add a test named "test", create a file called "test.ui", put it
into this directory. Then create the expected output file "test.txt",
put it into this directory too. You can create the initial version of
this file by invoking "./accessibility-dump --verbose test.ui". The
output will contain the expected text and can be copy/pasted into the
text file.