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).