tests/functional/asset: Verify downloaded size

tests/functional/asset: Verify downloaded size

If the server provides a Content-Length header, use that to verify the
size of the downloaded file. This catches cases where the connection
terminates early, and gives the opportunity to retry. Without this, the
checksum will likely mismatch and fail without retry.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Message-ID: <20250312130002.945508-3-npiggin@gmail.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>