Skip overview tests when semaphores are not implemented

[Julien-Elie]

Seen on Hurd:

Failed Set                 Fail/Total (%) Skip Stat  Failing Tests
-------------------------- -------------- ---- ----  ------------------------
overview/api                 27/75    36%    0   --  49-75
overview/buffindexed         21/21   100%    0    1  1-21
util/inndf                    2/6     33%    0    1  5-6

$ ./runtests -o overview/api
# buffindexed
cant create semaphore using ov-tmp/buffer: Function not implemented
failed to create semaphore for ov-tmp/buffer
buffindexed: ovinitdisks: cant create shmem for ov-tmp/buffer len 16384
Opening the overview database failed, cannot continue
Segmentation fault         ./runtests -o overview/api

$ ./runtests -o overview/buffindexed
1..21
cant create semaphore using ov-tmp/buffer: Function not implemented
failed to create semaphore for ov-tmp/buffer
buffindexed: ovinitdisks: cant create shmem for ov-tmp/buffer len 16384
Opening the overview database failed, cannot continue

$ ./inndf.t
# using buffindexed overview with etc/inn-bfx.conf
makehistory failed, unable to continue