tests: fix python style 'E302 expected 2 blank lines, found 1'

tests/bitarithm_timings/tests/01-run.py

tests/bloom_bytes/tests/01-run.py

tests/buttons/tests/01-run.py

tests/cbor/tests/01-run.py

tests/cpp11_condition_variable/tests/01-run.py

tests/cpp11_mutex/tests/01-run.py

tests/cpp11_thread/tests/01-run.py

tests/driver_ds1307/tests/01-run.py

tests/driver_grove_ledbar/tests/01-run.py

tests/driver_hd44780/tests/01-run.py

tests/driver_my9221/tests/01-run.py

tests/events/tests/01-run.py

tests/evtimer_msg/tests/01-run.py

tests/evtimer_underflow/tests/01-run.py

tests/float/tests/01-run.py

tests/gnrc_ipv6_ext/tests/01-run.py

tests/gnrc_ipv6_nib/tests/01-run.py

tests/gnrc_ipv6_nib_6ln/tests/01-run.py

tests/gnrc_ndp/tests/01-run.py

tests/gnrc_netif/tests/01-run.py