How can I test guitar effects box without a guitar?
As Andy said, the easiest way is to use a line out from your PC. However, there is an important caveat here:

The output impedance of the line out is drastically different from a guitar pickup output. Guitar pickups (and the associated tone/volume circuit) have a very high output impedance, often at least 10kΩ. With the volume and tone controls at midrange, the output impedance may be much higher, in the realm of 100kΩ. This is why most guitar effects are designed with a very high impedance input stage, typ. 250k-1MΩ. 

The consequence of this is that you might have a situation where you test a circuit with a PC line out and it works fine, but when using a guitar, it's almost inaudible or has massively reduced high end. This is because your circuit may present much to low of a load for the guitar to adequately drive, while the line out has no trouble.

So, as long as you make sure that you have a sufficiently high input impedance in your guitar effect circuits, a PC line output will be reasonably close. You will want to attenuate it; a guitar output might swing +/-1V or +/-2V with hot pickups, but the line out might be able to drive much more than that, so you should keep the volume pretty low on the PC for a fair representation. What's best is if you can record yourself playing a guitar directly into an audio interface without any post-processing, and play that back (yes, I know you don't have a guitar so this might not be an option), because most stuff you download will have at least some post-processing that changes the dynamics.