tests: Make scan_bss_expiration_count more robust with UML
Test case sequence "persistent_group_channel scan_bss_expiration_count"
was failing with UML when using time travel. This seemed to be because
there was no explicit wait to confirm that the AP has been fully
disabled before running the next scan. Work around this by verifying
that hostapd has terminated the BSS and waiting a bit after that before
proceeding.