Simulator terminal error is not cleared.

[microbit-robert]

To reproduce:

• Run simulator with no code.
• Create error, e.g., use undefined variable.
• Run simulator and see error output in simulator terminal.
• Clear error in code.
• Re-run simulator and see that error output is not cleared.

[microbit-matt-hillsdon]

We need to emit EVENT_SERIAL_RESET before a flash in the simulator device connection.

This matches the real device (which stops listening too, which we don't really need to do).