Update serial_test requirement from 0.5 to 0.6

Updates the requirements on [serial_test](https://github.com/palfrey/serial_test) to permit the latest version.
- [Release notes](https://github.com/palfrey/serial_test/releases)
- [Commits](palfrey/serial_test@v0.5.0...v0.6.0)

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

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

Author: dependabot[bot]

Cargo.toml

@@ -40,4 +40,4 @@ dirs = "4.0"
 kvm-bindings = "0.5"
 kvm-ioctls = "0.11"
 mmarinus = "0.2"
-serial_test = "0.5"
+serial_test = "0.6"