Fix runsc tests on VFS2 overlay.

- Check the sticky bit in overlay.filesystem.UnlinkAt(). Fixes
  StickyTest.StickyBitPermDenied.

- When configuring a VFS2 overlay in runsc, copy the lower layer's root
  owner/group/mode to the upper layer's root (as in the VFS1 equivalent,
  boot.addOverlay()). This makes the overlay root owned by UID/GID 65534 with
  mode 0755 rather than owned by UID/GID 0 with mode 01777. Fixes
  CreateTest.CreateFailsOnUnpermittedDir, which assumes that the test cannot
  create files in /.

- MknodTest.UnimplementedTypesReturnError assumes that the creation of device
  special files is not supported. However, while the VFS2 gofer client still
  doesn't support device special files, VFS2 tmpfs does, and in the overlay
  test dimension mknod() targets a tmpfs upper layer. The test initially has
  all capabilities, including CAP_MKNOD, so its creation of these files
  succeeds. Constrain these tests to VFS1.

- Rename overlay.nonDirectoryFD to overlay.regularFileFD and only use it for
  regular files, using the original FD for pipes and device special files. This
  is more consistent with Linux (which gets the original inode_operations, and
  therefore file_operations, for these file types from ovl_fill_inode() =>
  init_special_inode()) and fixes remaining mknod and pipe tests.

- Read/write 1KB at a time in PipeTest.Streaming, rather than 4 bytes. This
  isn't strictly necessary, but it makes the test less obnoxiously slow on
  ptrace.

Fixes #4407

PiperOrigin-RevId: 337971042

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