Comment # 38 on bug 1193124 from Takashi Iwai

Did you disable Secure Boot in your BIOS?  The test kernel won't boot when it's
enabled.

Also, when testing a new kernel, don't add the repository to zypper.  Instead,
just download kernel-default-5.15.*.rpm from the URL, and install it via zypper
directly like
  zypper install kernel-default-5.15.*.rpm

Sometimes you need --oldpackage option when installing, as the release number
might get reset.

Ditto for KMPs.  Download each rpm file, and install / uninstall each one
manually.