Fixed

Well, if it eases your pain, browsing other people's solutions and suggesting a similar tweak to another user, I remembered that instead of checking every odd number, just going for 6k+/-1 is much more efficient and edited my solution accordingly.

I suppose that after many similar problems (I did a lot with primes) one's mind goes fully autopilot ;)

Yeah, it actually is... Funny thing is, I found this same problem somewhere else and I used C and I did what you just said. I didn't give it much thought I suppose :P

Testing up to the square root of num is way more efficient, I presume :)

The test cases seem to be wrong. This:
test.assert_equals(rot13("test","grfg"))
test.assert_equals(rot13("Test","Grfg"))
Should look like this:
test.assert_equals(rot13("test"),"grfg")
test.assert_equals(rot13("Test"),"Grfg")
or else the test will always fail. But there's isn't any problem while submitting...