Fix rand_os warning in tests

[str4d]

Warnings were originally fixed in #134. This warning was introduced in #114, but it wasn't spotted because there was no merge conflict.

[nathan-at-least]

I am confused:

• Before merging this PR, it appears that master builds/tests successfully.
• Before merging this PR, it appears that zcash:master in https://github.com/zcash/librustzcash/blob/master/librustzcash/Cargo.toml#L31-L32 does not specify a rand_os dependency.
• This PR removes https://github.com/zcash/librustzcash/pull/144/files#diff-856d0c87bdf2a3f58e126d860ca3fd75L190 which is a use rand_os line.

How can all three of these be true? Shouldn't that use rand_os line cause building tests to fail, since it's not a declared dependency?

[nathan-at-least]

Hm, if the tests aren't build/executed for the code being patched, I would change my review to "Request Changes" to request they get added into the CI pipeline in this PR. If instead that code is tested but there's something I'm missing / confused about with dependencies or rust, then I would probably approve this comment and learn something new.

[str4d]

• Before merging this PR, it appears that master builds/tests successfully.

The tests all pass on this branch. The one failing step is Travis CI, which is trying to also run code coverage and as a result often runs over the 50-minute maximum execution time. Re-running the build is usually sufficient to have it succeed because then the build step is cached.

• Before merging this PR, it appears that zcash:master in https://github.com/zcash/librustzcash/blob/master/librustzcash/Cargo.toml#L31-L32 does not specify a rand_os dependency.

This PR is changing the zcash_client_backend crate, not the librustzcash crate. But this raises a good point: I forgot to remove the rand_os dependency from zcash_client_backend. I'll do so now.

[nathan-at-least]

I believe this resolves my previous confusion, and my new belief is that this dependency was declared, but no longer use'd anywhere. Is that true?

(Seems like an opportunity for a linter / unused-code style warning.)

[str4d]

I believe this resolves my previous confusion, and my new belief is that this dependency was declared, but no longer use'd anywhere. Is that true?

Correct. It was declared as a dev-dependency though, so zero effect on the main code. The tests wouldn't catch unused dev-dependencies, because they can be used for tests, doc-tests, examples, and benchmarks. There might be a clippy lint for it, but we're a while away from being able to add those to CI.

[Eirik0]

utACK