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

changelog/4941.feature.rst

@@ -0,0 +1,3 @@
+``pytester``'s ``Testdir`` sets ``$HOME`` and ``$USERPROFILE`` to the temporary directory.
+
+This ensures to not load configuration files from the real user's home directory.