Bug#970184: initramfs-tools: amd64-specific autopkgtests need to be skipped on !amd64

[Simon McVittie <smcv@debian.org>]

Package: initramfs-tools
Version: 0.137
Severity: important
X-Debbugs-Cc: debian-ci@lists.debian.org

The initramfs-tools autopkgtests now fail on arm64 (and would fail
on other non-amd64 architectures, if we had test runners for them)
because the test-dependencies are not installable. By default, the CI
infrastructure considers this to be a test failure.

Please add "Restrictions: skip-not-installable" to the amd64-* tests, so
that they will be skipped if the test-dependencies are uninstallable.

Alternatively, if you want to diagnose uninstallability on amd64
as a test failure, you could use "Restrictions: skippable", depend
on "linux-image-amd64 [amd64]", and make the test script exit 77 on
non-amd64 architectures.

Please see
https://salsa.debian.org/ci-team/autopkgtest/blob/master/doc/README.package-tests.rst
for more details.

Thanks,
    smcv