dev: add test-dev command for easier testing of packages (#2627)

@si14 you might know a better way to wire this up! lemme know if there's
something more clever here.

### Change Type

- [x] `internal` — Any other changes that don't affect the published
package[^2]

### Release Notes

- Adds easier testing command for individual packages.

apps/dotcom-worker/package.json

@@ -16,6 +16,7 @@
 		"dev-wrangler": "yarn run -T tsx ./scripts/dev-wrap.ts",
 		"report-size": "node scripts/report-size.js",
 		"test": "lazy inherit",
+		"test-dev": "yarn run -T jest --watch",
 		"test-coverage": "lazy inherit",
 		"lint": "yarn run -T tsx ../../scripts/lint.ts"
 	},