Remove error behavior for `cargo test`.

text/0000-security-advisories.md

@@ -149,8 +149,6 @@ and then release the versions that contain the security fix.
  Warning: bar vx.y.z (dependency of foo vx.y.z) is vulnerable. See https://crates.io/... for details.
  ```

- `cargo test` make those warnings hard errors.

- `cargo publish` will refuse to upload a crate if any version of a direct
  dependency satisfying the constraints in `Cargo.toml` is vulnerable. 
  Indirect dependencies should not trigger this behavior.
@@ -160,7 +158,7 @@ and then release the versions that contain the security fix.
  vulnerable, as another version satisfying that constraint may be.

The author of a crate that directly depends on a vulnerable crate may disable
these warnings/errors with a switch in their `Cargo.toml`. If `iron==0.4.x` has
these warnings with a switch in their `Cargo.toml`. If `iron==0.4.x` has
an advisory with the ID `deadbeef`, the dependent author may use the
`allow_vulnerable` parameter to disable all the above-described warnings and
errors for this vulnerability: