build(deps): update rstest requirement from 0.18 to 0.19 in /libDF

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@v0.18.0...v0.19.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

libDF/Cargo.toml

@@ -137,7 +137,7 @@ jemallocator = { version = "0.5.0", optional = true }
 
 [dev-dependencies]
 rand = "0.8"
-rstest = "0.18"
+rstest = "0.19"
 env_logger = "0.11"
 log = { version = "0.4", features = ["std"] }