testsome fails in the CircleCI multiplexer because the class JStackJUnitTask can't be found

As Andres de la Peña explained on CASSANDRA-16827, the problem might be in the flag -Dno-build-test=true that the multiplexer uses to not run build-test in every iteration, relying on that this has already been done by the build task. This is a performance optimisation that can be especially effective in fast tests.
It would be ideal to have the initial multiplexer behavior restored, but we can always run ant build-test at the beginning of every multiplexer runner while still not running it on every iteration. This seems enough to keep the multiplexer working but it requires some extra builds and therefore higher costs.

The goal of this ticket is to find the root of this failure and fix the multiplexer either by restoring its initial flow or adding ant build-test at the beginning of every multiplexer runner, if there is no more effective solution.