rk selftests, and kselftest units in rk test

[tamnd]

rk selftests builds the kernel's own selftests against a finished build, with the build's compiler or another, and installs them under <build>/selftests/kselftest. Each collection is made on its own with make -k, and the install uses make -i, because one program that needs a missing library otherwise drops the whole collection. What actually got installed is read back from kselftest-list.txt and written to selftests.json, since the make exit status says little.

rk test gains a kselftest kind. Each collection the row lists in rows.toml boots on its own, with run_kselftest.sh, the collection, and the shared libraries its programs need (found through PT_INTERP and DT_NEEDED) packed into the initramfs. Results become units named kselftest:<collection>:<program> and are graded like KUnit, reference against rucc. --rucc-selftests runs the rucc-built selftests on the rucc kernel when given, and the reference build otherwise. rk mixed takes --selftests so a failing selftest unit can be bisected too.

The nightly installs libcap-dev and libnuma-dev, builds the selftests with gcc-14 and with rucc, and runs the kselftest kind along with boot, smoke and KUnit.

Part of #6