Avoid creating arbitrary filenames for tmpdir on Testdir's constructor

Fixes #2751

_pytest/pytester.py

@@ -412,16 +412,8 @@ class Testdir:
     def __init__(self, request, tmpdir_factory):
         self.request = request
         self._mod_collections = WeakKeyDictionary()
-        # XXX remove duplication with tmpdir plugin
-        basetmp = tmpdir_factory.ensuretemp("testdir")
         name = request.function.__name__
-        for i in range(100):
-            try:
-                tmpdir = basetmp.mkdir(name + str(i))
-            except py.error.EEXIST:
-                continue
-            break
-        self.tmpdir = tmpdir
+        self.tmpdir = tmpdir_factory.mktemp(name, numbered=True)
         self.plugins = []
         self._savesyspath = (list(sys.path), list(sys.meta_path))
         self._savemodulekeys = set(sys.modules)

testing/test_collection.py

@@ -569,7 +569,6 @@ class Test_getinitialnodes(object):
         col = testdir.getnode(config, x)
         assert isinstance(col, pytest.Module)
         assert col.name == 'x.py'
-        assert col.parent.name == testdir.tmpdir.basename
         assert col.parent.parent is None
         for col in col.listchain():
             assert col.config is config