build.xml: Add tasks for coverage testing

Run ‘ant coverage-install’ once, then, to run the tests with coverage
attached and view the result, run ‘ant coverage’.

This took a while to set up correctly, but I’m really happy with how it
turned out – the generated reports (HTML) even include the source code,
each line annotated green/yellow/red/white depending on its coverage,
just like in EclEmma! Very nice.

(The actual coverage is less than expected, but keep in mind that the
Ceylon compiler inserts some synthetic statements that can never be
executed, e. g. in a switch without an `else` case, or to detect cyclic
initialization of toplevel objects.)