Added unit test for books getChildren() method (#6602)

* started unit tests for books

* added first unit test for getchildren()

* fixed dependencies and use tmpdir

* latest from master

extensions/notebook/package.json

@@ -347,6 +347,7 @@
 	"dependencies": {
 		"@jupyterlab/services": "^3.2.1",
 		"@types/js-yaml": "^3.12.1",
+		"@types/rimraf": "^2.0.2",
 		"decompress": "^4.2.0",
 		"error-ex": "^1.3.1",
 		"figures": "^2.0.0",
@@ -374,4 +375,4 @@
 		"vscode": "1.1.5"
 	},
 	"enableProposedApi": true
-}
+}