cp: restrict two test functions to linux/mac/win

[cakebaker]

This PR restricts two test functions to avoid the following "unused variable" warnings on FreeBSD:

warning: unused variable: `stdout_str`
      --> tests/by-util/test_cp.rs:3225:9
       |
  3225 |     let stdout_str = result.stdout_str();
       |         ^^^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_stdout_str`
       |
       = note: `#[warn(unused_variables)]` on by default
  
  warning: unused variable: `stdout_str`
      --> tests/by-util/test_cp.rs:3261:9
       |
  3261 |     let stdout_str = result.stdout_str();
       |         ^^^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_stdout_str`