caf_co_min and caf_co_max tests fail when num_images()==1

[rouson]

Building with GCC 14.2.0 on macOS 14.6.1 and executing the test suite in a single image reports failures in caf_co_min and caf_co_max:

git checkout 0.4.0-rc1
./install.sh
export GASNET_PSHM_NODES=1
./build/run-fpm.sh test

which produces the trailing following output:

Test that
    The prif_co_max subroutine computes the maximum
        elements across images with 2D arrays of strings
            Expected to be true
                User Message:
                    |all(expected == scramlet)|
    The prif_co_min subroutine computes the minimum
        elements across images with 2D arrays of strings
            Expected to be true
                User Message:
                    |all(expected == scramlet)|

2 of 59 cases failed
2 of 84 assertions failed

The test suite passes when GASNET_PSHM_NODES is 2 or 7.