std: mark common functions in test crate `pub(crate)`

This is not a library, so there's no reason for them to be `pub`.
Without doing this, compiling the test crates causes private dep
lint errors:

      error: type `PathBuf` from private dependency 'std' in public interface
        --> library/std/tests/common/mod.rs:26:5
         |
      26 |     pub fn join(&self, path: &str) -> PathBuf {
         |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
         |
         = note: `-D exported-private-dependencies` implied by `-D warnings`

      error: type `Path` from private dependency 'std' in public interface
        --> library/std/tests/common/mod.rs:31:5
         |
      31 |     pub fn path(&self) -> &Path {
         |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^

      error: could not compile `std` (test "create_dir_all_bare") due to 2 previous errors

This happens because Cargo passes `--extern 'priv:std=...` when
compiling the test crate.

I'm not sure if these warnings are desirable or not. They seem correct
in a very pedantic way (the dependency on `std` is not marked public,
since it's implicit), but also pointless (the test crate is not an API,
so who cares what it does).

library/std/tests/common/mod.rs

@@ -1,7 +1,5 @@
 #![allow(unused)]
 
-#![allow(exported_private_dependencies)]
-
 use std::env;
 use std::fs;
 use std::path::{Path, PathBuf};
@@ -22,15 +20,15 @@ pub(crate) fn test_rng() -> rand_xorshift::XorShiftRng {
 }
 
 // Copied from std::sys_common::io
-pub struct TempDir(PathBuf);
+pub(crate) struct TempDir(PathBuf);
 
 impl TempDir {
-    pub fn join(&self, path: &str) -> PathBuf {
+    pub(crate) fn join(&self, path: &str) -> PathBuf {
         let TempDir(ref p) = *self;
         p.join(path)
     }
 
-    pub fn path(&self) -> &Path {
+    pub(crate) fn path(&self) -> &Path {
         let TempDir(ref p) = *self;
         p
     }
@@ -51,7 +49,7 @@ impl Drop for TempDir {
 }
 
 #[track_caller] // for `test_rng`
-pub fn tmpdir() -> TempDir {
+pub(crate) fn tmpdir() -> TempDir {
     let p = env::temp_dir();
     let mut r = test_rng();
     let ret = p.join(&format!("rust-{}", r.next_u32()));