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)typeTestStdinInner = 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)structTestWriterLock<'a> {
| ^^^^^^^^^^^^^^
warning: `rustup` (lib) generated 4 warnings