Input: tests - fix use-after-free and refcount underflow in input_test_exit()

With CONFIG_DEBUG_SLAB=y:

        # Subtest: input_core
        1..3
    input: Test input device as /devices/virtual/input/input1
    8<--- cut here ---
    Unable to handle kernel paging request at virtual address 6b6b6dd7 when read
    ...
     __lock_acquire from lock_acquire+0x26c/0x300
     lock_acquire from _raw_spin_lock_irqsave+0x50/0x64
     _raw_spin_lock_irqsave from devres_remove+0x20/0x7c
     devres_remove from devres_destroy+0x8/0x24
     devres_destroy from input_free_device+0x2c/0x60
     input_free_device from kunit_try_run_case+0x70/0x94 [kunit]

Without CONFIG_DEBUG_SLAB=y:

	KTAP version 1
	# Subtest: input_core
	1..3
    input: Test input device as /devices/virtual/input/input1
    ------------[ cut here ]------------
    WARNING: CPU: 0 PID: 694 at lib/refcount.c:28 refcount_warn_saturate+0x54/0x100
    refcount_t: underflow; use-after-free.
    ...
    Call Trace: [<0037cad4>] dump_stack+0xc/0x10
     [<00377614>] __warn+0x7e/0xb4
     [<0037768c>] warn_slowpath_fmt+0x42/0x62
     [<001eee1c>] refcount_warn_saturate+0x54/0x100
     [<000b1d34>] kfree_const+0x0/0x20
     [<0036290a>] __kobject_del+0x0/0x6e
     [<001eee1c>] refcount_warn_saturate+0x54/0x100
     [<00362a1a>] kobject_put+0xa2/0xb6
     [<11965770>] kunit_generic_run_threadfn_adapter+0x0/0x1c [kunit]

As per the comments for input_allocate_device() and
input_register_device(), input_free_device() must be called only to free
devices that have not been registered.  input_unregister_device()
already calls input_put_device(), thus leading to a use-after-free.

Moreover, the kunit_suite.exit() method is called after every test case,
even on failures.  As the test itself already does cleanups in its
failure paths, this may lead to a second use-after-free.

Fix the first issue by dropping the call to input_allocate_device() from
input_test_exit().
Fix the second issue by making the cleanup code conditional on a
successful test.

Fixes: fdefcbdd

 ("Input: Add KUnit tests for some of the input core helper functions")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/957b3b309a44d39fb6e38b2a526b250f69ea3d2c.1683022164.git.geert+renesas@glider.be
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>