tests: add core18 only hooks test and fix running core18 only on classic

cmd/snap-confine/mount-support.c

@@ -702,8 +702,8 @@ void sc_populate_mount_ns(struct sc_apparmor *apparmor, int snap_update_ns_fd,
 	// TODO: fold this into bootstrap
 	setup_private_pts();
 
-	// setup quirks for specific snaps
-	if (distro == SC_DISTRO_CLASSIC) {
+	// setup quirks for specific snaps *only* for the old core snap
+	if (distro == SC_DISTRO_CLASSIC && sc_streq(base_snap_name, "core")) {
 		sc_setup_quirks();
 	}
 	// setup the security backend bind mounts

tests/lib/snaps/test-snapd-with-configure-core18/meta/hooks/configure

@@ -0,0 +1,3 @@
+#!/bin/sh -e
+
+touch "$SNAP_COMMON"/configure-ran

tests/lib/snaps/test-snapd-with-configure-core18/meta/snap.yaml

@@ -0,0 +1,5 @@
+name: test-snapd-with-configure-core18
+version: 1.0
+summary: Basic snap with a configure hook for core18
+description: A basic snap with a passthrough configure hook for core18
+base: core18

tests/main/core18-configure-hook/task.yaml

@@ -0,0 +1,31 @@
+summary: Check that snaps with configure work on core18 only
+
+systems: [-ubuntu-core-*]
+
+prepare: |
+    echo "Ensure empty state"
+    #shellcheck source=tests/lib/systemd.sh
+    . "$TESTSLIB/systemd.sh"
+    echo "Ensure all snaps are gone"
+    /usr/lib/snapd/snap-mgmt --purge
+
+    systemd_stop_units snapd.service
+    rm -f /var/lib/snapd/state.json
+    systemctl start snapd
+    snap wait system seed.loaded
+
+
+execute: |
+    #shellcheck source=tests/lib/snaps.sh
+    . "$TESTSLIB"/snaps.sh
+
+    echo "Install test snap"
+    install_local test-snapd-with-configure-core18
+
+    snap list | MATCH core18
+    if snap list | grep -E -q "^core "; then
+        echo "core got installed but shouldn't. test broken"
+        exit 1
+    fi
+
+    test -e /var/snap/test-snapd-with-configure-core18/common/configure-ran