Update rstest requirement from 0.19.0 to 0.21.0 (#12)

Updates the requirements on [rstest](https://github.com/la10736/rstest) to permit the latest version.
- [Release notes](https://github.com/la10736/rstest/releases)
- [Changelog](https://github.com/la10736/rstest/blob/master/CHANGELOG.md)
- [Commits](la10736/rstest@v0.19.0...v0.21.0)

---
updated-dependencies:
- dependency-name: rstest
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Cargo.toml

@@ -39,5 +39,5 @@ tokio = { version = "1.0", default-features = false, features = ["macros"] }
 
 [dev-dependencies]
 tokio = { version = "1.0", features = ["test-util", "rt-multi-thread"] }
-rstest = "0.19.0"
+rstest = "0.21.0"
 test-with = { version = "0.12.6", default-features = false }