add modules_per_string to cec_dc_adr_ac_system test fixture

[cwhanse]

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Corrects a test fixture used for the adr inverter model. The cec_dc_adr_ac_system fixture had a single 220W module paired with a 3kW inverter. The adr function returned nan for some valid input, due to unreasonably low DC:AC ratio.

[cwhanse]

For details, see this comment

Why is a NaN also in the first line, where the DC values look reasonable? Because the test system is malformed. The test system pairs a single 220W module with a 3kW inverter.