tests/functional: Convert the linux_initrd avocado test into a standalone test

While we're at it, update the SHA1 hashes to SHA256 hashes since
SHA1 should not be used anymore nowadays.

Message-ID:

tests/functional: Convert the linux_initrd avocado test into a standalone test

While we're at it, update the SHA1 hashes to SHA256 hashes since
SHA1 should not be used anymore nowadays.

Message-ID: <20240830133841.142644-32-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

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