Revisit mkdir/_ensure_supporting_files in cacheprovider

- cacheprovider: move call to _ensure_supporting_files

  This makes it less likely to have a race here (which is not critical),
  but happened previously probably with xdist, causing flaky coverage with
  `if not readme_path.is_file():` etc checks in
  `_ensure_supporting_files`, which has been removed in the `features`
  branch already.

src/_pytest/cacheprovider.py

@@ -121,10 +121,12 @@ class Cache(object):
                 cache_dir_exists_already = True
             else:
                 cache_dir_exists_already = self._cachedir.exists()
-            path.parent.mkdir(exist_ok=True, parents=True)
+                path.parent.mkdir(exist_ok=True, parents=True)
         except (IOError, OSError):
             self.warn("could not create cache path {path}", path=path)
             return
+        if not cache_dir_exists_already:
+            self._ensure_supporting_files()
         try:
             f = path.open("wb" if PY2 else "w")
         except (IOError, OSError):
@@ -133,9 +135,6 @@ class Cache(object):
             with f:
                 json.dump(value, f, indent=2, sort_keys=True)
 
-            if not cache_dir_exists_already:
-                self._ensure_supporting_files()
-
     def _ensure_supporting_files(self):
         """Create supporting files in the cache dir that are not really part of the cache."""
         readme_path = self._cachedir / "README.md"