tests: Make CHECK enforce 1 instead of just "!= 0"

[real-or-random]

This ensures that we don't omit the "== 1" in tests accidentally, and thus also strengthens existing tests in which it has been omitted.

We want to check "== 1" in particular for the return values of API functions, but it also makes sense in the case of internal functions.

If you really want to check "!= 0", you can still write it explicitly.

---

The second commit drops all the redundant "== 1"s. I tried to be careful to drop it only where it makes sense. Now, git grep "== 1" shows only instances where the 1 is supposed to be an actual integer and not a boolean. Still, this touches many lines, so I can drop this commit if you think it's too intrusive.

[jonasnick]

The second commit drops redundant "-1".

You mean "== 1"?

[theStack]

Concept ACK