kunit: Reset test->priv after each param iteration

If we run parameterized test that uses test->priv to prepare some
custom data, then value of test->priv will leak to the next param
iteration and may be unexpected. This could be easily seen if
we promote example_priv_test to parameterized test as then only
first test iteration will be successful:

$ ./tools/testing/kunit/kunit.py run \
--kunitconfig ./lib/kunit/.kunitconfig *.example_priv*

[ ] Starting KUnit Kernel (1/1)...
[ ] ============================================================
[ ] =================== example (1 subtest) ====================
[ ] ==================== example_priv_test ====================
[ ] [PASSED] example value 3
[ ] # example_priv_test: initializing
[ ] # example_priv_test: ASSERTION FAILED at lib/kunit/kunit-example-test.c:230
[ ] Expected test->priv == ((void *)0), but
[ ] test->priv == 0000000060dfe290
[ ] ((void *)0) == 0000000000000000
[ ] # example_priv_test: cleaning up
[ ] [FAILED] example value 2
[ ] # example_priv_test: initializing
[ ] # example_priv_test: ASSERTION FAILED at lib/kunit/kunit-example-test.c:230
[ ] Expected test->priv == ((void *)0), but
[ ] test->priv == 0000000060dfe290
[ ] ((void *)0) == 0000000000000000
[ ] # example_priv_test: cleaning up
[ ] [FAILED] example value 1
[ ] # example_priv_test: initializing
[ ] # example_priv_test: ASSERTION FAILED at lib/kunit/kunit-example-test.c:230
[ ] Expected test->priv == ((void *)0), but
[ ] test->priv == 0000000060dfe290
[ ] ((void *)0) == 0000000000000000
[ ] # example_priv_test: cleaning up
[ ] [FAILED] example value 0
[ ] # example_priv_test: initializing
[ ] # example_priv_test: cleaning up
[ ] # example_priv_test: pass:1 fail:3 skip:0 total:4
[ ] ================ [FAILED] example_priv_test ================
[ ] # example: initializing suite
[ ] # module: kunit_example_test
[ ] # example: exiting suite
[ ] # Totals: pass:1 fail:3 skip:0 total:4
[ ] ===================== [FAILED] example =====================

Fix that by resetting test->priv after each param iteration, in
similar way what we did for the test->status.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: David Gow <davidgow@google.com>
Cc: Rae Moar <rmoar@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>