⬆️ Update rstest requirement from 0.17 to 0.18

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@0.17.0...0.17.0)

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

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

Cargo.toml

@@ -15,7 +15,7 @@ markdown-it =  { version = "0.6", default-features = false }
 regex       = ">= 1.0.0, < 2"
 once_cell   = ">= 1.0.1, < 2"
 testing     = "0.33"
-rstest      = "0.17"
+rstest      = "0.18"
 # possible other packages for testing that are not actually used currently
 # indoc, rtest, insta