The use case you describe is definitely something we want to support in the future. It would probably fall into the category parallel and distributed build execution. At the moment, Gradle does not support this functionality. Are you already using parallel test execution to at least get the most out of the hardware resources on the machine executing your tests?
I cannot think of a straight forward way to implement this scenario with the capabilities we have in place right now. Dividing up the test cases by hash would be a way to shard the tests. The more complex problem is to dish out the test sets to remote machines and to collect/aggregate the test results.