sencha: add sencha-testbench targets Add targets that run the cheriot-rtos test suite on a sencha platform (cheriot-testbennch runs the test suite on a sail platform). Note this currently completes but the allocator test(s) fail because sencha's heap is too small. While here: - s/cheriot-tests-clean/cheriot-test-clean/ - use mkdir -p when creating tmp directories to avoid forced ordering Change-Id: I36a77e088031e4600b74bd4696277dfdc0fc101b