Tests: Execute compile features tests unconditionally.

Conditionally create a dummy test if there are no known features.
Features: FATAL_ERROR on compilers with no recorded features.

Users of the new target_compile_features command are expected to
check the existence of the CMAKE_CXX_COMPILE_FEATURES variable before
attempting to use it to require features.
Merge topic 'feature-absence-hard-error' into next

8d0b1cca Features: FATAL_ERROR on compilers with no recorded features.
447fbb3f Tests: Execute compile features tests unconditionally.