Sporadic cp test failure

[jgarzik]

Sporadically, this failure will appear during tests:

test cp::test_cp_perm ... FAILED

failures:

---- cp::test_cp_perm stdout ----
thread 'cp::test_cp_perm' panicked at tree/tests/cp/mod.rs:524:37:
assertion `left == right` failed
  left: 296
 right: 288
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    cp::test_cp_perm

test result: FAILED. 47 passed; 1 failed; 16 ignored; 0 measured; 0 filtered out; finished in 2.02s

error: test failed, to rerun pass `-p posixutils-tree --test integration`

And cargo clean followed by rebuild + retest always fixes this issue.