Today we select the port range to use in tests by taking the Gradle
worker ID mod 223. We now use significantly more than 223 Gradle workers
in each test run which means port ranges are re-used, resulting in
spurious failures.

This commit replaces the magic number 223 with a computation defined in
terms of the available ports and the number of ports to allocate per
worker. It also reduces the number of ports per worker to 30 to try and
reduce the frequency of port clashes.