Make tests work

* ci(sail): use npm for consistency

* ci(sail): use node:10, npx mocha for tests

* ci(sail): attempt one-line command

* ci(sail): use extra one line

* chore(npm): add test:mocha script

* ci(sail): use npm script

* ci(sail): this is why I need this extension

.sail.yml

···
 tasks:
   build:
-    image: node:carbon
+    image: node:10
     command:
       - npm
     args:
       - install
   test:
-    image: node:carbon
+    image: node:10
     command:
-      - yarn
+      - npm
     args:
-      - test
+      - run
+      - test:mocha

package.json

···
   "name": "vscode-cheer",
   "displayName": "Cheer",
   "description": "Have your editor cheer you on while you code!",
-  "version": "0.0.1",
+  "version": "1.0.0",
   "engines": {
     "vscode": "^1.29.0"
   },
···
   },
   "scripts": {
     "postinstall": "node ./node_modules/vscode/bin/install",
-    "test": "node ./node_modules/vscode/bin/test"
+    "test": "node ./node_modules/vscode/bin/test",
+    "test:mocha": "mocha -u tdd ./test/messages.test.js"
   },
   "publisher": "puregarlic",
   "author": {