textproc/libxmlb: Update to 0.3.21

Disable installed-tests. These seem harmless, but after the Jia Tan
long-con in xz-utils, best to just disable them since they are unused.
It's also kind of weird/sus to install random test data in general. This
seems to be "Hello, world." stuff for now, be could be more nefarious in
the future. Perhaps I'm just being a bit paranoid, but I don't see any
practical use for these test cases to be installed.