Bump tokio-postgres from 0.7.16 to 0.7.18 in /tests/client_tests/rust

Bumps [tokio-postgres](https://github.com/rust-postgres/rust-postgres) from 0.7.16 to 0.7.18.
- [Release notes](https://github.com/rust-postgres/rust-postgres/releases)
- [Commits](rust-postgres/rust-postgres@tokio-postgres-v0.7.16...tokio-postgres-v0.7.18)

---
updated-dependencies:
- dependency-name: tokio-postgres
  dependency-version: 0.7.18
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]