fix(filesource): make some constructs only available via the test feature

[rami3l]

Cherry-picked from #3803.

This PR has fixed the following warnings in https://github.com/rust-lang/rustup/actions/runs/8978258543/job/24658381207:

 warning: struct `TestStdinLock` is never constructed
    --> src/currentprocess/filesource.rs:47:8
     |
  47 | struct TestStdinLock<'a> {
     |        ^^^^^^^^^^^^^
     |
     = note: `#[warn(dead_code)]` on by default
  
  warning: type alias `TestStdinInner` is never used
    --> src/currentprocess/filesource.rs:68:17
     |
  68 | pub(crate) type TestStdinInner = Arc<Mutex<Cursor<String>>>;
     |                 ^^^^^^^^^^^^^^
  
  warning: struct `TestStdin` is never constructed
    --> src/currentprocess/filesource.rs:70:8
     |
  70 | struct TestStdin(TestStdinInner);
     |        ^^^^^^^^^
  
  warning: struct `TestWriterLock` is never constructed
     --> src/currentprocess/filesource.rs:182:19
      |
  182 | pub(super) struct TestWriterLock<'a> {
      |                   ^^^^^^^^^^^^^^
  
  warning: `rustup` (lib) generated 4 warnings