phd(?): we should test more kernels!

in https://github.com/oxidecomputer/propolis/issues/1173 I alluded to one of the interesting reasons about testing against a gamut of Linux distros, even though at the end of they day "they're all Linux": they've all got different kernels and different drivers.

the most recent Linux (and the most recent FreeBSD, and ..) won't be expressed by most distributions, so just testing distributions means there's a lot of time to miss "Linux change that Propolis/byhve don't play nice with". distributions are convenient single blobs we can boot and test, but in the ideal world of perfect testing we'd be able to build test Linux images with arbitrarily-recent kernels (similar to testing against, say OmniOS bloody or FreeBSD-STABLE)

there might be a nice interim point here, with a rolling release Linux distro? since just a kernel doesn't get you a userland, and mostly I'd have loved to learn about https://github.com/oxidecomputer/propolis/issues/1163 before 26.04 got cut..

@papertigers has fiddled with some scripts to assemble Alpine-like Linux images would would probably be suitable for running PHD tests against, and @lgfa29 has (afaik) used Packer to assemble a test image with fwts pre-installed. it might be possible to use either approach in this way, or maybe we do something different entirely!

Start by reading issues 1173 and 1163, then inspect the existing scripts mentioned for Alpine-like image assembly and the Packer approach with fwts. Determine how a test image with an arbitrarily recent kernel could be built and used for PHD tests; done means the image can boot and exercise those tests reliably.