test-execute/exec-dynamicuser-statedir.service: fix quoting

All backslashes that should be single in shell syntax need to be written as "\\" because
our parser will remove one level of quoting. Also, single quotes were doubly nested, which
cannot work.

Should fix the following message:
test-execute/exec-dynamicuser-statedir.service:16: Ignoring unknown escape sequences: "test $$(find / \( -path /var/tmp -o -path /tmp -o -path /proc -o -path /dev/mqueue -o -path /dev/shm -o -path /sys/fs/bpf -o -path /dev/.lxc \) -prune -o -type d -writable -print 2>/dev/null | sort -u | tr -d \\n) = /var/lib/private/quux/pief/var/lib/private/waldo"