Your Volvo 850's EVAP failure could have led to the check engine light turning on, however the check engine light being on would not have caused an EVAP leak test failure.
Here's how the EVAP Test work... Your Volvo has an EVAP hose which runs from the gas tank to the EVAP charcoal canister. The charcoal canister, usually located in one of the far corners of the engine compartment, is a container filled with charcoal pellets that absorb fuel vapors.
The EVAP test (LPFET) ensures that there are no leaks in the hose between the Gas Tank and Charcoal Canister... (and anywhere raw fuel exists). The technician must clamp the end of the hose leading to the canister and pressurize the gas tank with Nitrous Oxide via the EVAP test tool. The EVAP test tool will then calculate the drop in pressure due to any leaks. Technically there should be very little drop in pressure once the EVAP system (gas tank) is pressurized.
In order to fix your Volvo's EVAP failure it will be neccessary to locate the leak and repair the line. Depending on where the leak is located, this repair may be cheap or expensive, but the first step is to find where the leak is at.