tmpdir: fix temporary directories created with world-readable permissions

(Written for a Unix system, but might be applicable to Windows as well).

pytest creates a root temporary directory under /tmp, named
`pytest-of-<username>`, and creates tmp_path's and other under it.
/tmp is shared between all users of the system.

This root temporary directory was created with 0o777&~umask permissions,
which usually becomes 0o755, meaning any user in the system could list
and read the files, which is undesirable.

Use 0o700 permissions instead. Also for subdirectories, because the root
dir is adjustable.

src/_pytest/pathlib.py

@@ -205,7 +205,7 @@ def _force_symlink(
         pass
 
 
-def make_numbered_dir(root: Path, prefix: str) -> Path:
+def make_numbered_dir(root: Path, prefix: str, mode: int = 0o700) -> Path:
     """Create a directory with an increased number as suffix for the given prefix."""
     for i in range(10):
         # try up to 10 times to create the folder
@@ -213,7 +213,7 @@ def make_numbered_dir(root: Path, prefix: str) -> Path:
         new_number = max_existing + 1
         new_path = root.joinpath(f"{prefix}{new_number}")
         try:
-            new_path.mkdir()
+            new_path.mkdir(mode=mode)
         except Exception:
             pass
         else:
@@ -345,13 +345,17 @@ def cleanup_numbered_dir(
 
 
 def make_numbered_dir_with_cleanup(
-    root: Path, prefix: str, keep: int, lock_timeout: float
+    root: Path,
+    prefix: str,
+    keep: int,
+    lock_timeout: float,
+    mode: int,
 ) -> Path:
     """Create a numbered dir with a cleanup lock and remove old ones."""
     e = None
     for i in range(10):
         try:
-            p = make_numbered_dir(root, prefix)
+            p = make_numbered_dir(root, prefix, mode)
             lock_path = create_cleanup_lock(p)
             register_cleanup_lock_removal(lock_path)
         except Exception as exc: