test: metadata operations on unlinked open files and removed open directories (#11075)

The POSIX suites skirt this: pjdfstest's unlink/14.t covers only fstat and
pread on an unlinked descriptor — its driver has no fchmod at all and never
opens a directory. Pin the full rule in the FUSE integration suite:
ftruncate, fchmod, futimes, fstat, and the f*xattr calls keep working
between the removal of the last name and the final close, for a file after
unlink and a directory after rmdir, with nlink 0 and the changes visible to
a following fstat.

Claude-Session: https://claude.ai/code/session_01GYqLENjZzbV5hgt4L8cSAK

test/fuse_integration/go.mod

@@ -2,7 +2,10 @@ module seaweedfs-fuse-tests
 
 go 1.21
 
-require github.com/stretchr/testify v1.8.4
+require (
+	github.com/stretchr/testify v1.8.4
+	golang.org/x/sys v0.28.0
+)
 
 require (
 	github.com/davecgh/go-spew v1.1.1 // indirect