edge: [decomposedfs] concurrency test flaky

[butonic]

In https://drone.cernbox.cern.ch/cs3org/reva/8418/5/3 the decomposedfs concurrent createdir test failed:

------------------------------
• [FAILED] [1.060 seconds]
Decomposed
/drone/src/pkg/storage/utils/decomposedfs/decomposedfs_concurrency_test.go:36
  concurrent
  /drone/src/pkg/storage/utils/decomposedfs/decomposedfs_concurrency_test.go:53
    CreateDir
    /drone/src/pkg/storage/utils/decomposedfs/decomposedfs_concurrency_test.go:93
      [It] handle already existing directories
      /drone/src/pkg/storage/utils/decomposedfs/decomposedfs_concurrency_test.go:100

  Unexpected error:
      <*errors.withStack | 0xc00000f260>: {
          error: <*errors.withMessage | 0xc0005921a0>{
              cause: <*errors.errorString | 0xc000404890>{
                  s: "unable to acquire a lock on the file",
              },
              msg: "xattrs: Unable to lock file for read",
          },
          stack: [0xd31a26, 0xdf37b1, 0xdd89b8, 0xf623d8, 0x49ee61],
      }
      xattrs: Unable to lock file for read: unable to acquire a lock on the file
  occurred
  In [It] at: /drone/src/pkg/storage/utils/decomposedfs/decomposedfs_concurrency_test.go:115
------------------------------

and in https://drone.cernbox.cern.ch/cs3org/reva/8430/5/3

------------------------------
• [FAILED] [0.709 seconds]
Decomposed
/drone/src/pkg/storage/utils/decomposedfs/decomposedfs_concurrency_test.go:36
  concurrent
  /drone/src/pkg/storage/utils/decomposedfs/decomposedfs_concurrency_test.go:53
    CreateDir
    /drone/src/pkg/storage/utils/decomposedfs/decomposedfs_concurrency_test.go:93
      [It] handle already existing directories
      /drone/src/pkg/storage/utils/decomposedfs/decomposedfs_concurrency_test.go:100

  Unexpected error:
      <*errors.withStack | 0xc00000e4f8>: {
          error: <*errors.withMessage | 0xc00069e7a0>{
              cause: <*errors.errorString | 0xc0004b2200>{
                  s: "unable to acquire a lock on the file",
              },
              msg: "xattrs: Unable to lock file for read",
          },
          stack: [0xd31a26, 0xdf37b1, 0xdd89b8, 0xf623d8, 0x49ee61],
      }
      xattrs: Unable to lock file for read: unable to acquire a lock on the file
  occurred
  In [It] at: /drone/src/pkg/storage/utils/decomposedfs/decomposedfs_concurrency_test.go:115
------------------------------

[micbar]

Fixed by #3237