mod/integration_test: Introduce module

Introduce a module to simplify running integration tests using the
Unity test framework. It allows tests to be executed either
automatically on boot or via the debugger CLI.

Tests triggered via the debugger CLI are scheduled to run on the main
event loop.

Signed-off-by: Peter Hoyes <peter.hoyes@arm.com>
Signed-off-by: Athul P Satheesan <athul.ps@arm.com>