Reuse simulator between test bundles.

[njuliyu]

Currently bluepill recreate the simulator for each test bundle. We could save a lot of time if reusing the same simulator for more than one test bundles.

Proposals:

1. Add CLI option "not-delete-simulator" to bp command, so it won't delete the simulator when test cases finish normally, and return the device id to the parent bluepill process. By default bp command Always delete simulator.

2. Add CLI option "use-simulator" to bp command. The bluepill process pass the device id to bp command so bp can reuse the specified simulator without recreating a new one.

[oliverhu]

implemented in #54