simulator: Fix context single parameter

[efdx]

Summary

Fixes an incomplete feature in Commit: d32947517087214d28d71cdcbf3c02355526ab68 Feature/simulator addressing.

The single parameter was used incorrectly when ModbusServerContext was initialized. This caused the simulator to not work correctly when the configuration "device_id" was used. The context would wrap the dict in another, having completely incorrect, resulting in using an invalid object.

Fix it by defining single correctly.

The ModbusServerContext initialization looks like this:

    def __init__(self, slaves=None, single=True):
        """Initialize a new instance of a modbus server context.

        :param slaves: A dictionary of client contexts
        :param single: Set to true to treat this as a single context
        """
        self.single = single
        self._slaves = slaves or {}
        if self.single:
            self._slaves = {0: self._slaves}

So if single == True AND device_id is defined with an address, this will eventuall result in self._slaves being initialized as

self._slaves = {0: {<address>:<context>}}

instead of

self._slaves = {<address>:<context>}

Testing

1. Ran pytest, no regressions
2. Ran simulator with device_id defined.

[janiversen]

LGTM, thanks