Scripts: move the Startup and Shutdown script tests into tests/startup.rs

The tests now sit with the crate's other test modules under crates/script/src/tests/ and are declared in tests.rs. docs/scripting-parity.md points at the new path.

crates/script/src/tests.rs

@@ -17,6 +17,7 @@ mod include;
 mod panels;
 #[cfg(not(windows))]
 mod services;
+mod startup;
 mod tasks;
 mod ui;
 mod ui_more;