Add a test for thread-safe malloc ...

Add a test to repeatedly malloc and free from different threads to
test if these functions are thread safe.

Update thread stack size for NRF based targets in rtos tests.

Merge pull request #2248 from pan-/define_stack_stize_for_RTOS_threads_test ...

Define stack size of the threads spawned by RTOS threads test.

Define stack size of the threads spawned by this test. ...

This stack size is adjusted for NRF51 and NRF52 based targets.

Define stack size for NRF52.

Merge pull request #2241 from pan-/fix_stack_definitions ...

Bring missing stack definitions for various targets.

Remove double #else statement

Bring missing stack definitions for various targets. ...

Those definitions where available in libraries/test/rtos/mbed/* tests but
not in TESTS/mbedmicro-rtos-mbed/* tests.

Fix behavior when mbed-os RTOS tests are build while the system is single ...

threaded.

For mbed micro, if the MBED_RTOS_SINGLE_THREAD is defined then the
compilation of the test will fail with the info
	[NOT_SUPPORTED] test not supported

This patch bring this behavior to mbed-os RTOS tests.

Added tests for cv-qualifier c-objects in Callback class

Adding test frameworks and test sources ...

This commit adds the following test frameworks:

- `greentea-client` (https://github.com/ARMmbed/greentea-client)
  - This framework provides a key-value api for communicating with the
    greentea test tool (https://github.com/ARMmbed/greentea)
- `unity` (https://github.com/ARMmbed/unity)
  - This framework provides test assert macros that can be used when
    writing test cases
- `utest` (https://github.com/ARMmbed/utest)
  - This framework allows you to execute a series of test cases with
    reporting that works with the greentea test tool
    (https://github.com/ARMmbed/greentea)

The following changes were made when bringing these frameworks into the
tree:

- References to `mbed_drivers/mbed.h` within utest's tests were migrated
  to `mbed.h`
- The yotta file `module.json` was removed from `greentea-client` and
  `unity`
  - `coverage.json` was also removed from `greentea-client`
- `.gitignore` and `.gitattributes` were removed from `greentea-client`
- Apache 2.0 license files were removed from `greentea-client`

This also brings in a number of tests that have been newly written or ported from various sources:

- `TESTS/integration` - Very basic tests, used to check if testing frameworks are working correctly
- `TESTS/mbed_drivers` (Thanks @PrzemekWirkus!) - TESTS ported from mbed OS 3.0 (https://github.com/ARMmbed/mbed-drivers)
- `TESTS/mbedmicro-mbed` (Thanks @PrzemekWirkus!) - Tests that weren't covered by `TESTS/mbed_drivers` that currently live in `libraries/tests/mbed`
- `TESTS/mbedmicro-rtos-mbed` (Thanks @PrzemekWirkus!) - Ported tests that currently live in `libraries/tests/rtos/mbed`
- `TESTS/storage_abstraction` (Thanks @rgrover!) - Tests for the storage_abstraction hal