Unit tests: Create mbed-headers-filesystem ...

Create a CMake target mbed-headers-filesystem to separate FileSystem
headers from the generic mbed-headers-storage. Update tests to use it.

CMake: Require TEST_SOURCES in greentea CMake file ...

Assumption that greentea test file is always named main.cpp is
incorrect. Updated mbed_greentea_add_test() macro to make TEST_SOURCES
parameter compulsory, which is used to specify greentea test
file(s). This allows tests to use C, or have a different name.
Therefore also updated all pre-existing greentea test CMake files to
explicity add main.cpp to TEST_SOURCES.

CMake: rename greentea test macro

CMake: Add support for Filesystem Greentea tests ...

The Filesystem Greentea tests can now be built with CMake

Remove erroneous character from SPDX identifer ...

Erroneous '/' was appended to start of comment. This was causing CMake
warning during complition. Removes character from affected files.

Add SPDX license identifier to Arm files ...

Add license identifier to files which Arm owns the copyright to,
and contain either BSD-3 or Apache-2.0 licenses. This is to address
license errors raised by scancode analysis.

Move greentea test closure to library