tests: Disable fragm/rts_threshold after testing it

tests: Disable fragm/rts_threshold after testing it

The configurated fragmentation/RTS threshold value survives AP mode
interface restarts, so these values need to be explicitly cleared back
to default (disabled). This fixes an issue where some test cases could
not work correctly if fragmentation on the interface was enabled. For
example, this combination used to fail:
ap_fragmentation_open ap_hs20_fetch_osu

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>