test-execute: also mount tmpfs on /dev/shm

test-execute: also mount tmpfs on /dev/shm

Otherwise, if /dev/shm has a directory that cannot be accessible by
unprivileged user, then we cannot pick a dynamic user, and test service
may fail with unexpected error code:
---
Failed to enter shared memory directory /dev/shm/systemd-watch-bind-BqAGlN: Permission denied
exec-dynamicuser-supplementarygroups.service: Failed to update dynamic user credentials: Device or resource busy
exec-dynamicuser-supplementarygroups.service: Failed at step USER spawning /bin/sh: Device or resource busy
src/test/test-execute.c:885:test_exec_dynamicuser: exec-dynamicuser-supplementarygroups.service: can_unshare=no: exit status 217, expected 216
---

Follow-up for 4e032f654b94c2544ccf937209303766dfa66c24.