coverage.sh: add test for non-empty target directory

coverage.sh

@@ -52,7 +52,7 @@ if [ ! -e shared/mmdebstrap ] || [ mmdebstrap -nt shared/mmdebstrap ]; then
 fi
 
 starttime=
-total=111
+total=112
 i=1
 
 print_header() {
@@ -395,6 +395,32 @@ else
 	./run_null.sh
 fi
 
+print_header "mode=$defaultmode,variant=apt: fail installing to non-empty target directory"
+cat << END > shared/test.sh
+#!/bin/sh
+set -eu
+export LC_ALL=C.UTF-8
+mkdir /tmp/debian-chroot
+mkdir /tmp/debian-chroot/lost+found
+touch /tmp/debian-chroot/exists
+ret=0
+$CMD --mode=$defaultmode --variant=apt $DEFAULT_DIST /tmp/debian-chroot $mirror || ret=\$?
+rmdir /tmp/debian-chroot/lost+found
+rm /tmp/debian-chroot/exists
+rmdir /tmp/debian-chroot
+if [ "\$ret" = 0 ]; then
+	echo expected failure but got exit \$ret
+	exit 1
+fi
+END
+if [ "$HAVE_QEMU" = "yes" ]; then
+	./run_qemu.sh
+elif [ "$defaultmode" = "root" ]; then
+	./run_null.sh SUDO
+else
+	./run_null.sh
+fi
+
 print_header "mode=root,variant=apt: chroot directory not accessible by _apt user"
 cat << END > shared/test.sh
 #!/bin/sh