tests/empty-sources.list: only write out /etc/apt/sources.list if it doesn't exist yet

In the mmdebstrap autpkgtest, debian/tests/sourcesfilter will write out
/etc/apt/sources.list before the test's setup-hook is run. Thus, the
test's setup-hook will overwrite the contents of the sources.list that
debian/tests/sourcesfilter set up, which will end up pulling in the
wrong packages.

Thus, only write out our own /etc/apt/sources.list if it does not yet
exist. If it does exist, nothing needs to be done.

tests/empty-sources.list

@@ -3,6 +3,6 @@ set -eu
 export LC_ALL=C.UTF-8
 trap "rm -f /tmp/debian-chroot.tar" EXIT INT TERM
 printf '' | {{ CMD }} --mode={{ MODE }} --variant=apt \
-	--setup-hook='echo "deb {{ MIRROR }} {{ DIST }} main" > "$1"/etc/apt/sources.list' \
+	--setup-hook='test -e "$1"/etc/apt/sources.list || echo "deb {{ MIRROR }} {{ DIST }} main" > "$1"/etc/apt/sources.list' \
 	{{ DIST }} /tmp/debian-chroot.tar -
 tar -tf /tmp/debian-chroot.tar | sort | diff -u tar1.txt -