PM / sleep: Fix test_suspend after sleep state rework

When passing "test_suspend=mem" to the kernel:

    PM: can't test 'mem' suspend state

and the suspend test is not run.

Commit 406e7938 ("PM / sleep: System sleep state selection
interface rework") changed pm_labels[] from a contiguous NULL-terminated
array to a sparse array (with the first element unpopulated), breaking
the assumptions of the iterator in setup_test_suspend().

Iterate from PM_SUSPEND_MIN to PM_SUSPEND_MAX - 1 to fix this.

Fixes: 406e7938 (PM / sleep: System sleep state selection interface rework)
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>