The test kernel is not fixing the issue. The actual error seems to be entirely different now (as if we could not execute the test application (nsenter).

Log file added below:

+ ./reproduce-lp-1656121.sh
+ snap install --devmode snapd-hacker-toolbelt
snapd-hacker-toolbelt 0.6 from 'zygoon' installed
+ /usr/lib/snapd/snap-discard-ns snapd-hacker-toolbelt
+ perl -n -e /-- cursor: (.+)/ && print $1
+ tail -n 1
+ journalctl --show-cursor
+ cursor=s=f99269010cfc407fa45669c97647431c;i=36a;b=f0a02ed5b1bc42fa939eecd92100e10d;m=135bccf;t=5477b9acae4c4;x=5c3ca07400f5d0e
+ echo 0
+ echo 1
+ echo noquiet
sh: echo: I/O error
+ snapd-hacker-toolbelt.busybox sh -c id && nsenter -m/proc/1/ns/mnt /bin/true
uid=0(root) gid=0(root) groups=0(root)
sh: nsenter: Permission denied
+ echo Test failed. Please look at the error messages below.
Test failed. Please look at the error messages below.
+ cat
+ journalctl --after-cursor=s=f99269010cfc407fa45669c97647431c;i=36a;b=f0a02ed5b1bc42fa939eecd92100e10d;m=135bccf;t=5477b9acae4c4;x=5c3ca07400f5d0e
-- Logs begin at Wed 2017-02-01 18:44:31 CET, end at Wed 2017-02-01 18:44:47 CET. --
Feb 01 18:44:47 autopkgtest kernel: AppArmor: cap_mac_admin? 1
Feb 01 18:44:47 autopkgtest kernel: AppArmor: policy locked? 0
Feb 01 18:44:47 autopkgtest snapd-hacker-toolbelt.busybox[1165]: cmd.go:111: DEBUG: restarting into "/snap/core/current/usr/bin/snap"
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined attached to new label
Feb 01 18:44:47 autopkgtest kernel: apparmor: clearing unsafe personality bits. /usr/lib/snapd/snap-confine label=/usr/lib/snapd/snap-confine
Feb 01 18:44:47 autopkgtest kernel: 
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest kernel: AppArmor: aa_setprocattr_changehat: (pid 1178) Magic 0x0 count 0 hat 'mount-namespace-capture-helper'
Feb 01 18:44:47 autopkgtest kernel: apparmor: clearing unsafe personality bits. /usr/lib/snapd/snap-exec label=snap.snapd-hacker-toolbelt.busybox
Feb 01 18:44:47 autopkgtest kernel: 
Feb 01 18:44:47 autopkgtest kernel: AppArmor: aa_label_alloc (ffffa1a457e1b700)
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest kernel: AppArmor: aa_label_alloc (ffffa1a457e1b180)
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest kernel: AppArmor: unconfined exec no attachment
Feb 01 18:44:47 autopkgtest audit[1212]: AVC apparmor="ALLOWED" operation="exec" profile="snap.snapd-hacker-toolbelt.busybox" name="/usr/bin/nsenter" pid=1212 comm="busybox" requested_mask="x" denied_mask="x" fsuid=0 ouid=0 target="snap.snapd-hacker-toolbelt.busybox//null-/usr/bin/nsenter"
+ false
-----