wifi: stop running supplicant before tests

The first test would always fail because it tries to
use system-initiating supplicant process.

Bug: 155257934
Test: atest VtsHalWifiSupplicantV1_3TargetTest
Change-Id: I79b69ec04faab5e8f2af0e5e3a9b1cc4c3aa0c36
2 files changed