Tests/RunCMake: Move documentation to a README.rst
Tests/RunCMake: Add function to run a specified command-line

Add a 'run_cmake_command' function that can be used by tests to run a
given command-line and check the results rather than always running a
CMake configuration process.  This can be used in the future to test
'cmake -E' for example.
Merge topic 'improve-Tests-RunCMake' into next

d4ca30ae Tests/RunCMake: Add function to run a specified command-line
392a6553 Tests/RunCMake: Move documentation to a README.rst