tests: add test to check layouts are removed properly

As in some corner cases this was not the case and the applications
could not even start.

tests/main/layout-remove/task.yaml

@@ -0,0 +1,36 @@
+summary: Ensure that removing a layout works as expected
+
+details: |
+  This test installs a test snap that uses layout declarations and
+  then refreshes it with a new version that removes one of the
+  layouts.
+
+# No core22 snap for i386
+systems: [-ubuntu-*-32]
+
+prepare: |
+  "$TESTSTOOLS"/snaps-state install-local test-snapd-layout
+
+execute: |
+  snap pack test-layout-v1
+  snap pack test-layout-v2
+  snap install --dangerous test-layout_1.0_all.snap
+  # Check layouts. Note that the mount namespace is created in the first run.
+  test-layout.test -c "test -d /var/test_tmpfs"
+  test-layout.test -c "test -d /var/lib/test_common"
+  test-layout.test -c "test -d /var/lib/test_data"
+  # Layouts have been configured
+  MATCH test_tmpfs < /run/snapd/ns/snap.test-layout.fstab
+  MATCH test_common < /run/snapd/ns/snap.test-layout.fstab
+  MATCH test_data < /run/snapd/ns/snap.test-layout.fstab
+
+  # Refreshing will update the mount namespace
+  snap install --dangerous test-layout_1.1_all.snap
+  # Should run fine
+  test-layout.test -c "test -d /var/test_tmpfs"
+  not test-layout.test -c "test -d /var/lib/test_common"
+  not test-layout.test -c "test -d /var/lib/test_data"
+  # There is no trace of the removed layouts
+  MATCH test_tmpfs < /run/snapd/ns/snap.test-layout.fstab
+  not MATCH test_common < /run/snapd/ns/snap.test-layout.fstab
+  not MATCH test_data < /run/snapd/ns/snap.test-layout.fstab

tests/main/layout-remove/test-layout-v1/bin/test

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

tests/main/layout-remove/test-layout-v1/meta/snap.yaml

@@ -0,0 +1,19 @@
+name: test-layout
+version: 1.0
+summary: Test layouts
+description: 'Test layout changes'
+base: core22
+confinement: strict
+grade: devel
+
+layout:
+  /var/lib/test_data:
+    bind: $SNAP_DATA/test_data
+  /var/lib/test_common:
+    bind: $SNAP_COMMON/test_common
+  /var/test_tmpfs:
+    type: tmpfs
+
+apps:
+  test:
+    command: bin/test

tests/main/layout-remove/test-layout-v2/bin/test

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

tests/main/layout-remove/test-layout-v2/meta/snap.yaml

@@ -0,0 +1,15 @@
+name: test-layout
+version: 1.1
+summary: Test layouts
+description: 'Test layout changes'
+base: core22
+confinement: strict
+grade: devel
+
+layout:
+  /var/test_tmpfs:
+    type: tmpfs
+
+apps:
+  test:
+    command: bin/test