MLECO-3747: Minor changes to Catch2 inclusion

Catch2, included for tests on native target, is now added
using FetchContent to download it at configuration stage.
Minor improvements added to prevent name clashes if this
project is used as a submodule in other projects where
Catch2 is already a named dependency/target.

Change-Id: I6f0ff02704e54a5c4cefe499ac80d434cf89b463
Signed-off-by: Kshitij Sisodia <kshitij.sisodia@arm.com>