Wifi: Make sure that VTS tests start with a clean state

In current code base, The SetUp() method in wifi VTS tests does not
reset Wifi state. Hence the first Wifi test runs with unspecified state.
Following test cases are not impacted due to that TearDown() takes care
of resetting the Wifi state.

This commit adds the stopWifi() method to the SetUp() so all test cases
start from the same Wifi state.

Bug: 145312303
Test: Run all wifi VTS tests
Test: atest <VTS test module name>
Change-Id: I63d7c0d27f4ceb713dda3804786a199dc2274be5