Updated FindGaudi.cmake not to make CMake configuration impossible
when Gaudi is missing, and to also correctly forward find_package(...)
flags (like REQUIRED).

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.