Add `mbed-tools test` command

[rwalton-arm]

We want a test command to enable running greentea tests (with CTest) easily.

• Allow user to specify test configuration #297
• Add the Greentea test command to all relevant documentation #295
• CTest integration: Add option to list all tests #294
• Add support to run individual and groups of Greentea tests #296
• Allow unittests for mbed-tools test command to be run #298