Tests: Run RunCMake.target_compile_features unconditionally.

It is so far testing only cases which are fatal regardless of
recorded features.
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

a69c13b0 Features: FATAL_ERROR on compilers with no recorded features.
8a074538 Tests: Execute compile features tests unconditionally.
597bb72e Tests: Run RunCMake.target_compile_features unconditionally.