xz-utils 5.6.0 has been uploaded to unstable. A changed behaviour of
`xz' is now that mutlti threaded compress/ decompression is now enabled
by default. This in turn leads to warnings if the requested amount of
memory exceeds the available amount. A snippet from
https://ci.debian.net/packages/u/unar/testing/armel/43341293/
| 115s foo.tar.xz: Tar in XZ
| 115s   foo/  (dir)... OK.
| 115s   foo/bar  (4 B)... OK.
| 115s   foo/baz  (4 B)... OK.
| 115s Successfully extracted to "foo".
| 116s autopkgtest [10:53:54]: test tar: -----------------------]
| ▾ test tar: test results
| 116s autopkgtest [10:53:54]: test tar:  - - - - - - - - - - results - - - - - - - - - -
| 116s tar                  FAIL stderr: xz: Reduced the number of threads from 16 to 8 to not exceed the memory usage limit of 1400 MiB
| 116s autopkgtest [10:53:54]: test tar:  - - - - - - - - - - stderr - - - - - - - - - -

Allowing output on stderr would be a possible tix.

Sebastian