pytester: use temporary HOME with spawn

Followup to https://github.com/pytest-dev/pytest/issues/4956.

changelog/4956.feature.rst

@@ -0,0 +1 @@
+pytester's ``testdir.spawn`` uses ``tmpdir`` as HOME/USERPROFILE directory.

src/_pytest/pytester.py

@@ -1233,7 +1233,13 @@ class Testdir(object):
         if sys.platform.startswith("freebsd"):
             pytest.xfail("pexpect does not work reliably on freebsd")
         logfile = self.tmpdir.join("spawn.out").open("wb")
-        child = pexpect.spawn(cmd, logfile=logfile)
+
+        # Do not load user config.
+        env = os.environ.copy()
+        env["HOME"] = str(self.tmpdir)
+        env["USERPROFILE"] = env["HOME"]
+
+        child = pexpect.spawn(cmd, logfile=logfile, env=env)
         self.request.addfinalizer(logfile.close)
         child.timeout = expect_timeout
         return child

testing/test_pytester.py

@@ -559,3 +559,26 @@ def test_popen_default_stdin_stderr_and_stdin_None(testdir):
     assert stdout.splitlines() == [b"", b"stdout"]
     assert stderr.splitlines() == [b"stderr"]
     assert proc.returncode == 0
+
+
+def test_spawn_uses_tmphome(testdir):
+    import os
+
+    tmphome = str(testdir.tmpdir)
+
+    # Does use HOME only during run.
+    assert os.environ.get("HOME") != tmphome
+
+    p1 = testdir.makepyfile(
+        """
+        import os
+
+        def test():
+            assert os.environ["HOME"] == {tmphome!r}
+        """.format(
+            tmphome=tmphome
+        )
+    )
+    child = testdir.spawn_pytest(str(p1))
+    out = child.read()
+    assert child.wait() == 0, out.decode("utf8")