Add find_if tests

[tcbrindle]

From #193 by @Guekka:

Sidenote: while writing this, I've noticed find_if doesn't seem tested?

Embarrassingly, it's true, we don't have specific tests for find_if and find_if_not (although they're used in the implementation of filter and various other places, so we know they work...)

The lack of tests should be fixed.