Test case 755 "check if smart supports hinting for RRECOMMENDS" is not effective; pn-buildlist is determined without the involvement of smart. If you want to test this properly you need to build an image with PACKAGE_CLASSES = "rootfs_rpm" (the default) and check that the recommended package has been installed. This could be done using buildhistory i.e. looking to see that the package is listed in depends.dot with style=dotted (indicating it's a recommend relationship).
Could we use bitbake-whatchanged for this in stead? I believe it will be faster as there is no need for a rebuild.
No, that won't help either. You need to be testing something where smart is actually getting run. The test must either look at the contents of an rpm-based image on the host side to ensure it contains a certain package because of a recommends relationship (see my original comment); or alternatively the test could install a package at runtime using smart that has a recommends relationship on another package and check that the other package also gets installed.
Test case modified. Now run-time testing is required. https://bugzilla.yoctoproject.org/tr_show_case.cgi?case_id=755