Add basic notebook model tests (#3396)

- Ported from the extension
- Only adding tests that related to the internally implemented functionality, not to anything provider-specific.

package.json

@@ -90,6 +90,7 @@
     "@types/mocha": "2.2.39",
     "@types/sanitize-html": "^1.18.2",
     "@types/semver": "5.3.30",
+    "@types/should": "^13.0.0",
     "@types/sinon": "1.16.34",
     "@types/winreg": "^1.2.30",
     "asar": "^0.14.0",
@@ -148,8 +149,10 @@
     "queue": "3.0.6",
     "remap-istanbul": "^0.6.4",
     "rimraf": "^2.2.8",
+    "should": "^13.2.3",
     "sinon": "^1.17.2",
     "source-map": "^0.4.4",
+    "temp-write": "^3.4.0",
     "tslint": "^5.9.1",
     "typemoq": "^0.3.2",
     "typescript": "2.9.2",