tests: add reproducer for lp 1871591

The test is comprised of a core18-based app with two layout items for
/etc/foo file bind-mounts, several interfaces and two hooks. So far it
does not reproduce the issue.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

tests/regression/lp-1871591/task.yaml

@@ -0,0 +1,10 @@
+summary: regression test for UNKNOWN 
+prepare: |
+    snap pack test-snapd-app
+    snap install --dangerous test-snapd-app_1_all.snap
+execute: |
+    test-snapd-app.sh -c true
+    snap install --dangerous test-snapd-app_1_all.snap
+    test-snapd-app.sh -c true
+debug: |
+    snap remove test-snapd-app

tests/regression/lp-1871591/test-snapd-app/bin/sh

@@ -0,0 +1,3 @@
+#!/bin/sh
+PS1='$ '
+exec /bin/sh "$@"

tests/regression/lp-1871591/test-snapd-app/meta/hooks/install

@@ -0,0 +1,2 @@
+#!/bin/sh
+echo "hook: $0 $*" >>$SNAP_COMMON/hook.log

tests/regression/lp-1871591/test-snapd-app/meta/hooks/post-refresh

@@ -0,0 +1,2 @@
+#!/bin/sh
+echo "hook: $0 $*" >>$SNAP_COMMON/hook.log

tests/regression/lp-1871591/test-snapd-app/meta/snap.yaml

@@ -0,0 +1,18 @@
+name: test-snapd-app
+version: 1
+architectures: [all]
+base: core18
+confinement: strict
+layout:
+  /etc/mke2fs.conf:
+    bind-file: $SNAP_COMMON/etc/mke2fs.conf
+  /etc/mtools.conf:
+    bind-file: $SNAP_COMMON/etc/mtools.conf
+apps:
+    sh:
+        command: bin/sh
+        plugs:
+            - home
+            - mount-observe
+            - network-observe
+            - ssh-public-keys