cmake: disable Catch2 tests when Catch2 is unavailable

debhelper on noble passes -DFETCHCONTENT_FULLY_DISCONNECTED=ON, so CPM
cannot fetch Catch2 and silently skips it, leaving no Catch2 targets
behind and breaking the generate step. Fall back to WITH_CATCH2=OFF
with a warning instead.

Signed-off-by: Sun Yuechi <sunyuechi@iscas.ac.cn>