Issue Description:

The Swift test suite, when run on Android host devices, does not rate-limit itself. That is, if the test suite is configured to use 8 cores (the number of cores on the host machine), it will attempt to run 8 tests in parallel on the connected Android device, even if that device only has 4 cores.

As @gribozavr mentions in this GitHub comment, executing tests on device without limiting concurrency will usually oversubscribe the device and will lead to spurious failures (out of memory issues etc.) That's because the number of device cores is typically much smaller than the number of cores on the host where lit is running.

@gribozavr suggests adding a persistent server that would be responsible for device communication, and have %target-run be a thin client that just asks the server to run a certain command. The server would throttle (or possibly load balance over multiple devices!) as appropriate.