Fix usage of pytester with doctests (#6802)

Use `request.node.name` instead of `request.function.__name__`:
`request.function` is `None` with `DoctestItem`s.

testing/test_pytester.py

@@ -89,6 +89,29 @@ def test_testdir_runs_with_plugin(testdir) -> None:
     result.assert_outcomes(passed=1)
 
 
+def test_testdir_with_doctest(testdir):
+    """Check that testdir can be used within doctests.
+
+    It used to use `request.function`, which is `None` with doctests."""
+    testdir.makepyfile(
+        **{
+            "sub/t-doctest.py": """
+        '''
+        >>> import os
+        >>> testdir = getfixture("testdir")
+        >>> str(testdir.makepyfile("content")).replace(os.sep, '/')
+        '.../basetemp/sub.t-doctest0/sub.py'
+        '''
+    """,
+            "sub/__init__.py": "",
+        }
+    )
+    result = testdir.runpytest(
+        "-p", "pytester", "--doctest-modules", "sub/t-doctest.py"
+    )
+    assert result.ret == 0
+
+
 def test_runresult_assertion_on_xfail(testdir) -> None:
     testdir.makepyfile(
         """