synthrepo: when adding files, ensure new path is not a directory

synthrepo: synthesized dates must be positive, fit in 32-bit signed ints

discovery: test coverage for issue4438 / 86c35b7ae300 / 73cfaa348650 The randomness in the discovery protocol made this problem hard to reproduce. The test mocks random.sample to make sure we hit the problem every time. The set iteration order also made the output unstable ... but with the issue fixed, it is stable.