itest: add missing build tag

[jharveyb]

Avoid building itests when building for a release.

[guggero]

I don't think we should add build tags to each file, as that makes it a bit harder to use (for example, the itest files aren't compiled anymore when running the unit tests, which is sometimes handy to detect compilation errors early).
Because the release binary shouldn't have any reference to the itest package at all, it will not be built by default.
So something is wrong and the offender seems to be proof/aperture.go. That needs to be moved to the itest package, then we resolve the original issue.

[guggero]

Replaced by #378.