add testing

Author: filipesilva

public/docs/ts/latest/cookbook/third-party-lib.jade

@@ -252,9 +252,33 @@ code-example(language="json").
 
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-  ## Testing
+  ## Testing libraries
 
-  content
+  While testing is always important, it's **especially** important in libraries because consumer
+  applications might break due to bugs in libraries.
+
+  But the fact that a library is consumed by another application is also what makes it hard to test.
+
+  To properly test a library, you need to have an integration test.
+  An integration test is to libraries what a end-to-end test is to applications.
+  It tests how an app would install and use your library.
+
+  The **QuickStart Library seed** includes a directory called `integration` containing a standalone
+  app that consumes your built library in both JIT and AOT modes, with end-to-end tests to verify
+  it works.
+
+  To run the integration tests, do `npm run integration` which does the following:
+  - Build your library.
+  - Enter the integration app's directory.
+  - Install dependencies.
+  - Test the app in JIT mode.
+  - Build the app in AOT mode.
+  - Test the app in AOT mode.
+
+  Running integration tests gives you greater confidence that your library is properly built.
+
+  In addition to integration tests, you can also run unit tests in watch mode via `npm run test`,
+  or single-run via `npm run test:once`.
 
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