CMake Robustness Improvements, main branch (2023.10.08.)

Added various CMake "robustness improvements".

• Updated FindGaudi.cmake not to make CMake configuration impossible when Gaudi is missing, and to also correctly handle find_package(...) flags (like REQUIRED) when Gaudi is missing;
• Reduced the message level in some checks in AtlasLibraryFunctions.cmake, to only print a warning when Gaudi is missing in a function call that absolutely needs Gaudi;
• Finally, made the atlas_add_test(...) function call a bit more robust against using -NOTFOUND strings in its parameters. Which could, in some cases, confuse the existing security checks in the code.

With these in place I am able to generate Makefiles for the Athena project even in "super catastrophic" circumstances, when none of tdaq-common, tdaq or Gaudi are available for the build. The build itself of course fails for almost every package at that point, but at least the build can be attempted.

This, like many recent CMake changes, is related to the efforts around ATLINFR-5071.