Run tests with --boxed

When using XCB extensions you need to query the extension before making
requests to it. 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/