Fix issue #1218: Arrange, Act, Assert pattern (#1221)

* add AAA pattern

* reformat comment

* use Testing in readme tags in stead of Unit Test

arrange-act-assert/README.md

@@ -0,0 +1,30 @@
+---
+layout: pattern
+title: Arrange/Act/Assert
+folder: arrange-act-assert
+permalink: /patterns/arrange-act-assert/
+categories: Idiom
+tags:
+ - Testing
+---
+
+## Also known as
+Given/When/Then
+
+## Intent
+The Arrange/Act/Assert (AAA) is a pattern for organizing unit tests.
+It breaks tests down into three clear and distinct steps:
+1. Arrange: Perform the setup and initialization required for the test.
+2. Act: Take action(s) required for the test.
+3. Assert: Verify the outcome(s) of the test.
+
+## Applicability
+Use Arrange/Act/Assert pattern when
+
+* you need to structure your unit tests so they're easier to read, maintain, and enhance. 
+
+## Credits
+
+* [Arrange, Act, Assert: What is AAA Testing?](https://blog.ncrunch.net/post/arrange-act-assert-aaa-testing.aspx)
+* [Bill Wake: 3A – Arrange, Act, Assert](https://xp123.com/articles/3a-arrange-act-assert/)
+* [Martin Fowler: GivenWhenThen](https://martinfowler.com/bliki/GivenWhenThen.html)