Relax file size comparison criteria

[bulli92]

Comment from Martin Evaldson

1. I've found that comparing checksums can be too precise for comparing output images generated via ESMValTool. When running the RobotFramework for testing I used the number of digits in the size of the file and this would actually change on occasion, e.g., from five digits to four. If running on the same machine, using the same software stack and the same modules loaded it might work but it is probably more convenient (for ESMVal, not for easytest in general) to have some leeway and instead look at the reference/test folder difference.