Update serial_test requirement from 0.6 to 0.8 (#19)

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@v0.6.0...v0.8.0)

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

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Author: dependabot[bot]

embedded-profiling-proc-macros/Cargo.toml

@@ -15,7 +15,7 @@ syn = { version = "1.0", features = ["full"] }
 
 [dev-dependencies]
 embedded-profiling = { path = "../embedded-profiling", version = "^0.3", features = ["proc-macros"] }
-serial_test = "0.6"
+serial_test = "0.8"
 
 [lib]
 proc-macro=true

embedded-profiling/Cargo.toml

@@ -14,7 +14,7 @@ fugit = ">=0.3.3,<0.4"
 embedded-profiling-proc-macros = { path = "../embedded-profiling-proc-macros", version = "0.2", optional = true }
 
 [dev-dependencies]
-serial_test = "0.6"
+serial_test = "0.8"
 
 [features]
 default = []