coverage.sh: add test for installing onto existing file

coverage.sh

@@ -48,7 +48,7 @@ if [ ! -e shared/mmdebstrap ] || [ mmdebstrap -nt shared/mmdebstrap ]; then
 fi
 
 starttime=
-total=99
+total=100
 i=1
 
 print_header() {
@@ -687,6 +687,25 @@ else
 	./run_null.sh SUDO
 fi
 
+print_header "mode=root,variant=apt: fail installing to existing file"
+cat << END > shared/test.sh
+#!/bin/sh
+set -eu
+export LC_ALL=C.UTF-8
+touch /tmp/exists
+ret=0
+$CMD --mode=root --variant=apt $DEFAULT_DIST /tmp/exists $mirror || ret=\$?
+if [ "\$ret" = 0 ]; then
+	echo expected failure but got exit \$ret
+	exit 1
+fi
+END
+if [ "$HAVE_QEMU" = "yes" ]; then
+	./run_qemu.sh
+else
+	./run_null.sh SUDO
+fi
+
 # to test foreign architecture package installation we choose a package which
 #   - is not part of the native installation set
 #   - does not have any dependencies