Add terminateRequest to the test DebugClient

[DanTup]

I think it's essentially a copy/paste of the disconnectRequest function:

// Existing
public disconnectRequest(args?: DebugProtocol.DisconnectArguments): Promise<DebugProtocol.DisconnectResponse> {
    return this.send('disconnect', args);
}

// New
public terminateRequest(args?: DebugProtocol.TerminateArguments): Promise<DebugProtocol.TerminateResponse> {
    return this.send('terminate', args);
}