thermal: testing: zone: Flush work items during cleanup

thermal: testing: zone: Flush work items during cleanup

To prevent freed module code from being executed during the thermal
testing module unload, make it add a dedicated workqueue for thermal
testing work items and flush it in thermal_testing_exit().

Fixes: f6a034f2df42 ("thermal: Introduce a debugfs-based testing facility")
Link: https://sashiko.dev/#/patchset/20260605185212.2491144-1-sam.moelius%40trailofbits.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/1959388.tdWV9SEqCh@rafael.j.wysocki
[ rjw: Make variable d_command static ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>