Skip one_device.t if /dev/shm isn't a mountpoint
Skip one_device.t if /dev/shm isn't a mountpoint

If /dev/shm isn't a separate device to its parent directory, skip using
it to test limiting a search to a single device.  If /dev/shm isn't a
mountpoint there's no obvious alternative for checking Ag doesn't
descend into one, so just skip the test entirely.

Fixes #864.

JFLarvoire/the_silver_searcher,sebgod/the_silver_searcher,lizh06/the_silver_searcher,ggreer/the_silver_searcher,nodakai/the_silver_searcher,k-takata/the_silver_searcher,sebgod/the_silver_searcher,godbyk/the_silver_searcher,msys2/the_silver_searcher,avih/the_silver_searcher,JFLarvoire/the_silver_searcher,JFLarvoire/the_silver_searcher,msys2/the_silver_searcher,lizh06/the_silver_searcher,avih/the_silver_searcher,k-takata/the_silver_searcher,avih/the_silver_searcher,bhaak/the_silver_searcher,godbyk/the_silver_searcher,ggreer/the_silver_searcher,lizh06/the_silver_searcher,sebgod/the_silver_searcher,bhaak/the_silver_searcher,lizh06/the_silver_searcher,JFLarvoire/the_silver_searcher,JFLarvoire/the_silver_searcher,msys2/the_silver_searcher,JFLarvoire/the_silver_searcher,lecheel/the_silver_searcher,ggreer/the_silver_searcher,k-takata/the_silver_searcher,avih/the_silver_searcher,godbyk/the_silver_searcher,msys2/the_silver_searcher,godbyk/the_silver_searcher,lecheel/the_silver_searcher,k-takata/the_silver_searcher,bhaak/the_silver_searcher,nodakai/the_silver_searcher,nodakai/the_silver_searcher,JFLarvoire/the_silver_searcher,lecheel/the_silver_searcher,sebgod/the_silver_searcher,lecheel/the_silver_searcher,bhaak/the_silver_searcher,nodakai/the_silver_searcher,bhaak/the_silver_searcher,avih/the_silver_searcher,godbyk/the_silver_searcher,k-takata/the_silver_searcher,nodakai/the_silver_searcher,ggreer/the_silver_searcher,msys2/the_silver_searcher