Run tests with --boxed

When using XCB extensions you need to query the extension before making
requests to it. (This is referred to in the XCB documentation as the
"extension cache".) Older versions of cairo sometimes don't do this, so the
first test will pass, but the second test will fail because cairo is caching
request ids from the old connection (an example wth an xtrace is [1]). To work
around this, we run each test in a separate process via --boxed, which forces
cairo to ask XCB about the extensions for each test.

[1]: http://paste.ubuntu.com/8320590/