Add notifier tests (#4064)

Proposal:
1. Use anyhow for tests and avoid unwrap
2. Extract a helper for starting a test instance of codex

codex-rs/core/tests/common/Cargo.toml

@@ -1,12 +1,13 @@
 [package]
+edition = "2024"
 name = "core_test_support"
 version = { workspace = true }
-edition = "2024"
 
 [lib]
 path = "lib.rs"
 
 [dependencies]
+anyhow = { workspace = true }
 codex-core = { workspace = true }
 serde_json = { workspace = true }
 tempfile = { workspace = true }