Set the minimum required version of CMake high enough to avoid the
warning for CMAKE_LEGACY_CYGWIN_WIN32. The warning appears on stderr
and breaks the expected output matching.
Add cmCommand::Disallowed helper to check the setting of a policy that
disallows the command. Add a RunCMake.DisallowedCommands test
placeholder. Add a Help/policy/DISALLOWED_COMMAND.txt file for
inclusion by each policy document to avoid duplication of the common
text.