focal: autopkgtests: d/t/mount fails with 'error: cannot download snap "dragonboard-kernel": no snap revision available as specified'

[Impact]

The autopkgtests for Focal are failing due to d/t/mount,
because (un)specified snap revisisions aren't available:

  error: cannot download snap "dragonboard-kernel": no snap revision available as specified

  error: cannot download: no snap revision available as specified: "pc", "pc-kernel"

This isn't a big deal, but might report false-negatives
for autopkgtests regressions for dependencies (example:
SRU for, multipath-tools bug 2039719 comment #12), which
takes some time to check and handle.

The fix is to specify the snap revisions used by model
.assertion files, which match those in the last stable
ubuntu-core 16 (16.04.5) image [1], using its .manifest
files:

https://cdimage.ubuntu.com/ubuntu-core/16/stable/current/

[Test Plan]

Check that autopkgtests (specifically, d/t/mount test)
passes for ubuntu-image on Focal.

Expected:

    2185s autopkgtest [08:55:55]: @@@@@@@@@@@@@@@@@@@@ summary
    ...
    2185s mount PASS

Actual:

    4393s autopkgtest [18:40:46]: @@@@@@@@@@@@@@@@@@@@ summary
    ...
    4393s mount FAIL non-zero exit status 1

[Regression Potential]

The changes are limited to `d/t/mount` before running `ubuntu-image`,
and providing downloaded snap files to it, so any regressions would
likely manifest in the test (which is already broken) probably with
snap-related error messages.

[Other Info]

The fix is not required / does not apply to Jammy and later, since
the autopkgtests changed there, and `d/t/mount` is no longer available.

...

The focal git branch in LP/GH is well ahead of the Ubuntu Archive,
and no longer matches the `debian/tests` structure with `d/t/mount`,
thus the package is being directly uploaded to Ubuntu without a MR.

...

Autopkgtests results against a PPA with the fix:

https://autopkgtest.ubuntu.com/results/autopkgtest-focal-mfo-ubuntu-image-focal-dep8-v2/?format=plain

[Original Error Log]

2690s autopkgtest [18:12:23]: test mount: [-----------------------
2690s ==========> dragonboard-model.assertion
2690s Warning: for backwards compatibility, `ubuntu-image` falls back to `ubuntu-image snap` if no subcommand is given
2690s DEBUG:ubuntu-image:-> [ 0] make_temporary_directories
2690s DEBUG:ubuntu-image:-> [ 1] prepare_gadget_tree
2690s DEBUG:ubuntu-image:-> [ 2] prepare_image
2691s WARNING: proceeding to download snaps ignoring validations, this default will change in the future. For now use --validation=enforce for validations to be taken into account, pass instead --validation=ignore to preserve current behavior going forward
2691s 2023/11/16 18:12:23.373431 store_download.go:142: no host system xdelta3 available to use deltas
2691s error: cannot download snap "dragonboard-kernel": no snap revision available as specified
2691s ERROR:ubuntu-image:COMMAND FAILED: snap prepare-image --channel=stable /tmp/autopkgtest.hdThZq/build.Bhc/src/debian/tests/models/dragonboard-model.assertion /tmp/autopkgtest.hdThZq/autopkgtest_tmp/dragonboard-model/unpack
2691s ERROR:ubuntu-image:Full debug traceback follows
2691s Traceback (most recent call last):
2691s File "/tmp/autopkgtest.hdThZq/build.Bhc/src/ubuntu_image/assertion_builder.py", line 26, in prepare_image
2691s snap(self.args.model_assertion, self.unpackdir,
2691s File "/tmp/autopkgtest.hdThZq/build.Bhc/src/ubuntu_image/helpers.py", line 142, in snap
2691s run(cmd, stdout=None, stderr=None, env=os.environ)
2691s File "/tmp/autopkgtest.hdThZq/build.Bhc/src/ubuntu_image/helpers.py", line 123, in run
2691s proc.check_returncode()
2691s File "/usr/lib/python3.8/subprocess.py", line 448, in check_returncode
2691s raise CalledProcessError(self.returncode, self.args, self.stdout,
2691s subprocess.CalledProcessError: Command '['snap', 'prepare-image', '--channel=stable', '/tmp/autopkgtest.hdThZq/build.Bhc/src/debian/tests/models/dragonboard-model.assertion', '/tmp/autopkgtest.hdThZq/autopkgtest_tmp/dragonboard-model/unpack']' returned non-zero exit status 1.
2691s Traceback (most recent call last):
2691s File "/tmp/autopkgtest.hdThZq/build.Bhc/src/debian/tests/mount", line 50, in <module>
2691s with open(image_file_list, 'r', encoding='utf-8') as fp:
2691s FileNotFoundError: [Errno 2] No such file or directory: '/tmp/autopkgtest.hdThZq/autopkgtest_tmp/dragonboard-model/image-list.txt'
2691s autopkgtest [18:12:24]: test mount: -----------------------]