cp,tail: fix warnings in tests on Android

[cakebaker]

This PR fixes some "unused import" and "unused variable" warnings in tests on Android as shown in https://github.com/uutils/coreutils/actions/runs/6756912757/job/18366894422?pr=5494#step:8:2114:

warning: unused import: `rlimit::Resource`
  --> tests/by-util/test_cp.rs:28:5
   |
28 | use rlimit::Resource;
   |     ^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(unused_imports)]` on by default

warning: unused import: `std::path::Path`
  --> tests/by-util/test_tail.rs:29:5
   |
29 | use std::path::Path;
   |     ^^^^^^^^^^^^^^^

warning: unused import: `tail::text`
  --> tests/by-util/test_tail.rs:37:5
   |
37 | use tail::text;
   |     ^^^^^^^^^^

warning: unused import: `std::fmt::Write`
 --> tests/by-util/test_tee.rs:6:5
  |
6 | use std::fmt::Write;
  |     ^^^^^^^^^^^^^^^


Waiting for /sdcard/tests.probe: /sdcard/tests.log: 1 file pulled, 0 skipped. 1.4 MB/s (20059 bytes in 0.014s)

warning: unused variable: `result`
    --> tests/by-util/test_cp.rs:3444:9
     |
3444 |     let result = ts
     |         ^^^^^^ help: if this is intentional, prefix it with an underscore: `_result`
     |
     = note: `#[warn(unused_variables)]` on by default

The warning for tee should be fixed in #5473