When more than one value is given to the NAMES option this command by
default will consider one name at a time and search every directory for
it. Add a NAMES_PER_DIR option to tell this command to consider one
directory at a time and search for all names in it.
Previously this command was tested only implicitly as part of larger
tests. Add a RunCMake.find_program test to cover find_program cases
specifically and independently.