Some of the test cases where using INTERWORKING_SELECT internally
without using scan_for_bss like the helper functions did. Add explicit
scan_for_bss calls to make the test cases less likely to fail due to
missing BSSes in scan results. This could cause false failure reports
when runnign under heavy load with parallel-vm.sh.
Signed-off-by: Jouni Malinen <j@w1.fi>