Currently you have to remember to add: TEST_SUITES_append = " systemd" to enable systemd tests. The systemd tests should be added automatically for systemd images.
Sent patch for review: https://patchwork.openembedded.org/series/4911/#
Merged in oe-core 2e6ba167bee7c7b42b1e0e96eb5a7be2eb0fd2c0.