‘Testable’ is spelled ‘testable’ because it points to the ‘ability to be tested’. Incredibly enough, when the ‘able to …’ part is not a complete word, you have to write an i instead of the a. Here is a usable explanation of the rules.

In my experience, this is definately definitely one of the least intuitive spelling rules in English. Did someone ever tell me about this in school?

(Foor de rekkort, i af olweeys bien in feyvor of a moor fonetikal weey of spelling ennieweey )