Fix Wallbox price unit

[hesselonline]

Fix wrong currency value in the tests for wallbox

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[hesselonline]

@MartinHjelmare regarding comment #82039 (comment), I fix this in this PR. The api does provide the relevant 3-letter codes (I just made a mistake in te test ;-)). Also, regarding your other question, this is just the price per kWh (doesn't increase, cannot be summarized).

[MartinHjelmare]

If the unit is eg EUR / kWh we shouldn't use the currency device class. Please remove the currency device class and update the sensor unit to the correct one, including /kWh.

[hesselonline]

@MartinHjelmare I pushed a change that removes the deviceclass and creates a dynamic UoM -> [currency]/kWh

[MartinHjelmare]

Thanks!