Add TESTC-3X test sequences to produce nested hosts from different 8.X upgrades

[vxgmichel]

Draft PR to document the install sequences corresponding to the cards TESTC-33, TESTC-34, and TESTC-35.

More specifically, they produce the following cache keys:

UEFI_830_83NIGHTLY='install.test::Nested::boot_upg[uefi-830-83nightly-host1-iso-ext]-vm1-$COMMIT_SHA'
BIOS_830_83NIGHTLY='install.test::Nested::boot_upg[bios-830-83nightly-host2-iso-ext]-vm1-$COMMIT_SHA'
UEFI_821_83NIGHTLY='install.test::Nested::boot_upg[uefi-821.1-83nightly-host1-iso-ext]-vm1-$COMMIT_SHA'
BIOS_821_83NIGHTLY='install.test::Nested::boot_upg[bios-821.1-83nightly-host2-iso-ext]-vm1-$COMMIT_SHA'
UEFI_81_83NIGHTLY='install.test::Nested::boot_upg[uefi-81-83nightly-host1-iso-ext]-vm1-$COMMIT_SHA'
BIOS_81_83NIGHTLY='install.test::Nested::boot_upg[bios-81-83nightly-host2-iso-ext]-vm1-$COMMIT_SHA'

They can then be used with commands such as:

uv run pytest --nest $L0_HOST --hosts="cache://$UEFI_81_83NIGHTLY" tests/misc/test_access_links.py

or:

uv run jobs.py run postinstall-with-tls "cache://$UEFI_81_83NIGHTLY,cache://$BIOS_81_83NIGHTLY" --nest $L0_HOST