What term describes the quality of code which makes it easily testable?

There is this JS library I'm using which has a singleton design. When I attempt to use the library in many different tests, the library's global state from the previous test is contaminating the next test. I would say "the problem here is that the library is using the Singleton pattern, thus is not easily testable".

What term describes the "testable" quality that this library is lacking?

I want to say, that this library should be refactored so that it:

• becomes "easily testable?"
• is "not a singleton?"
• can "handle plurality?"
• can "handle 'arity'?"

What is the correct way for me to describe the quality that I would like this library to incorporate such that it is more easily testable?