pytester: testdir: set $HOME to tmpdir

This avoids loading user configuration, which might interfere with test
results, e.g. a `~/.pdbrc.py` with pdb++.

Also sets USERPROFILE, which will be required with Python 3.8 [1].

1: https://bugs.python.org/issue36264

src/_pytest/pytester.py

@@ -469,6 +469,8 @@ class Testdir(object):
         os.environ["PYTEST_DEBUG_TEMPROOT"] = str(self.test_tmproot)
         os.environ.pop("TOX_ENV_DIR", None)  # Ensure that it is not used for caching.
         os.environ.pop("PYTEST_ADDOPTS", None)  # Do not use outer options.
+        os.environ["HOME"] = str(self.tmpdir)  # Do not load user config.
+        os.environ["USERPROFILE"] = os.environ["HOME"]
         self.plugins = []
         self._cwd_snapshot = CwdSnapshot()
         self._sys_path_snapshot = SysPathsSnapshot()