coremqtt-mock: Add subset of functionalities

This commit expands mock coverage across the repo in order to test
core_mqtt_agent.c and freertos-command-pool.c.
Mock support is in the form of modifying CMakeLists.txt in
connectivity, aws_iot, and security components.
All .h and .c files needed by core-mqtt are also added.

This commit renames the mock directories.
Now mocks for libraries are contained in `library_mocks`
subdirectories, and mocks for integration code in `integration_mocks`
subdirectories.
Only these folder names have changed, they are still in the same parent
directories.
This decision was made to improve clarity of what each folder contains.

This commit also fixes missing `BUILD_TESTING` usages in
`CMakeLists.txt`.

This commit moves mocks for `applications/helpers` from the test
subdirectories into `applications/helpers//mocks`.

Updates mocks: freertos_kernel.

Adds (from scratch) mock support for:
security/mbedtls,
security/trusted_firmware-m,
connectivity/iot_vsocket,
connectivity/freertos_plus_tcp,
connectivity/backoff_algorithms,
coremqtt_agent aws_iot module.

Additionally, mocks application-specific config files for testing
core_mqtt_agent.
Store application-specific config files in `tests/mocks` directory to
avoid cluttering the `tests/` directory with non-test files.
Also adds a CMakeLists.txt file for this directory.

Also adds build_unit_test directory to .gitignore.

Signed-off-by: Reuben Cartwright <Reuben.Cartwright@arm.com>