Move content to testing_intro.md

Move testing overview content and diagram to testing overview page to resolve issue #718

Author: Amanda Butler

docs/tools/testing/testing_intro.md

@@ -2,6 +2,22 @@
 
 Testing is a critical step in the development process. The Arm Mbed ecosystem offers several tools to help you test your code.
 
+Traditional software testing is defined into three main levels: unit testing, integration testing and system testing. These levels are often pictured as a pyramid to indicate the amount of testing for each level.
+
+```
+^ Testing level
+|
+|       /\
+|      /  \       System testing
+|     /----\
+|    /      \     Integration testing
+|   /--------\
+|  /          \   Unit testing
+| /------------\
+|
+*-------------------> Amount of tests
+```
+
 You can use unit testing tools to build and run Mbed OS unit tests. Each unit test is built into an isolated test executable using a separate build system and build tools native to the development machine without any hardware or software dependencies. You can use the tool with Mbed CLI using the keyword `test` with `--unittests` flag.
 
 Greentea, Icetea, `htrun` and `mbed-ls` are testing tools written in Python. Greentea tests serve as functional unit tests in C++, as well as integration tests for complex use cases that execute on microcontrollers. The Icetea test tool adds more support for interoperability testing by making it easier to handle multiple devices and external service during testing. Arm Mbed CLI has a verb `test` that drives these tools to form a testing system. These comprise our automated testing framework for Mbed OS development.