Tests loopbackFileSystem.Utimens() and loopbackfile.Utimens()
at 1-second precision.

The exising TestUtimesNano() test only works on Linux
because it relies on syscall.UtimesNano(), which is not
available on Darwin. The new tests call the Utimens()
functions directly, bypassing FUSE, and work on all
platforms.

Because Darwin does not have syscall.UtimesNano(),
getting the Utimens() implementation right is hard.

The tests currently fail on Darwin, underlining the
need for them ( https://github.com/rfjakob/gocryptfs/issues/229 ):

$ go test ./fuse/nodefs
[...]
--- FAIL: TestLoopbackFileUtimens (0.00s)
	helpers.go:51: mtime has changed: 1525384186 -> 1073
	helpers.go:70: atime has changed: 1525291058 -> 1073
[...]

$ go test ./fuse/pathfs
--- FAIL: TestLoopbackFileSystemUtimens (0.00s)
	helpers.go:51: mtime has changed: 1525384379 -> 1073
	helpers.go:70: atime has changed: 1525291058 -> 1073
[...]