chore: start a local server for testing when run dev #17868 (#17882)

README.md

@@ -45,6 +45,9 @@ Execute the instructions in the root directory of the echarts:
 npm install
 
 # Rebuild source code immediately in watch mode when changing the source code.
+# It opens the `./test` directory and you may open `-cases.html` to get the list
+# of all test cases.
+# If you wish to create a test case, run `npm run mktest:help` to learn more.
 npm run dev
 
 # Check correctness of TypeScript code.

package.json

@@ -46,7 +46,7 @@
     "build:lib": "node build/build.js --prepublish",
     "build:extension": "node build/build.js --type extension",
     "dev:fast": "node build/build-i18n.js && node build/dev-fast.js",
-    "dev": "npm run dev:fast",
+    "dev": "npx -y concurrently -n build,server \"npm run dev:fast\" \"npx -y http-server -c-1 -s -o test\"",
     "prepublish": "npm run build:lib",
     "release": "npm run build:lib && npm run build:i18n && npm run build && npm run build:esm && npm run build:extension",
     "help": "node build/build.js --help",