Update rstest requirement from 0.18.1 to 0.19.0 (extendr#734)

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@v0.18.1...v0.19.0)

---
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>

extendr-api/Cargo.toml

@@ -23,7 +23,7 @@ serde = { version = "1.0", features = ["derive"], optional = true }
 
 [dev-dependencies]
 extendr-engine = { path = "../extendr-engine" }
-rstest = "0.18.1"
+rstest = "0.19.0"
 
 [features]