PM / sleep: add TEST_PLATFORM support for freeze state

Invoke freeze_enter() after suspend_test(TEST_PLATFORM) being invoked.

So when setting /sys/power/pm_test to "platform", it can be used to
check if freeze state is working well after all devices are suspended
and before processors are blocked,

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>