Add documentation about icetea testcases

Author: Jarno Lamsa

TEST_APPS/testcases/README.md

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+Icetea tests
+============
+
+This folder contains all the test cases done with Icetea residing in `mbed-os`.
+The tests are divided in to subfolders and each subfolder contains a set of testcases.
+The subfolder has a description of all the testcases it contains.
+
+Testcases
+---------
+
+Current testcases:
+
+-   [netsocket](https://github.com/ARMmbed/mbed-os/blob/master/TEST_APPS/testcases/netsocket)
+-   [example](https://github.com/ARMmbed/mbed-os/blob/master/TEST_APPS/testcases/example)

TEST_APPS/testcases/example/README.md

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+Example tests
+=============
+
+This folder contains example tests for Icetea
+The test located under this folder is dependent of the application [exampleapp](https://github.com/ARMmbed/mbed-os/blob/master/TEST_APPS/device/exampleapp)
+The exampleapp is disabled by default, to be able to run the test_cmdline with the exampleapp, either remove the preprocessor macro from exampleapp.cpp or add `-DICETEA_EXAMPLE_ENABLED` to the mbed test command
+
+Testcases
+---------
+
+### test_cmdline
+
+**Description:**
+Send command line commands to target over serial interface.
+This test introduces the Icetea testcase structure to the user.
+
+**Test steps:**
+Send "echo hello world" to the target.
+Target sends "hello world" back to the host machine.
+Send help to the target.
+Target prints out the command line commands the application supports.
+
+**Expected result:**
+The test exits without timeouts.

TEST_APPS/testcases/netsocket/README.md

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+Netsocket tests
+===============
+
+This folder contains netsocket tests for Icetea
+The tests located under this folder are dependent of the application [socket_app](https://github.com/ARMmbed/mbed-os/blob/master/TEST_APPS/device/socket_app)
+
+The test cases under this folder are defined in [Network Socket test plan](https://github.com/ARMmbed/mbed-os/blob/master/TESTS/netsocket/README.md)