I'm an Android developer and I'm looking for a test phone, since I don't want my main phone plugged in and charging all day. I noticed that there are many "Google Locked" android phones for sale on eBay for very cheap. Can you test apps on a google locked phone? I don't care about connecting to wifi, mobile data, etc. I just need to be able to plug-in via USB and run my app on the phone to see how my app looks at a different resolution, performance, etc.
No... Assuming by "Google Locked" you mean it has an enforced Factory Reset Protection which requires the previous owner's (or last active) Google account credentials. When this is active you cannot get to the launcher or perform any actions on the device until it is "unlocked" using the proper Google credentials. It is "locked", is essentially a non-functional device, and you cannot get past the Google Credentials screen in any way.
Some further information that might be applicable in our FRP base question, How to bypass “Verify your account” (Factory Reset Protection)