Add Testdir.__str__ to return str(self.tmpdir)

I just expected this with `monkeypatch.setenv("PYTHONPATH", str(testdir))`,
wondering why it was not picked up correctly.
`

src/_pytest/pytester.py

@@ -518,6 +518,9 @@ class Testdir(object):
     def __repr__(self):
         return "<Testdir %r>" % (self.tmpdir,)
 
+    def __str__(self):
+        return str(self.tmpdir)
+
     def finalize(self):
         """Clean up global state artifacts.