use pytest.mark.xfail; add changelog entry

Author: antonblr

changelog/7393.bugfix.rst

@@ -0,0 +1 @@
+unittest ``@expectedFailure`` marker with unexpected success outcome and if ``xfail_strict=False`` now translated to pytest's "XPASS" (was: "FAILED")

src/_pytest/unittest.py

@@ -30,8 +30,6 @@
 from _pytest.python import PyCollector
 from _pytest.runner import CallInfo
 from _pytest.skipping import skipped_by_mark_key
-from _pytest.skipping import Xfail
-from _pytest.skipping import xfailed_key
 
 if TYPE_CHECKING:
     import unittest
@@ -279,10 +277,7 @@ def addUnexpectedSuccess(
         msg = "Unexpected success"
         if reason:
             msg += f": {reason}"
-        strict = self.config.getini("xfail_strict")
-        self._store[xfailed_key] = Xfail(
-            reason=msg, run=True, strict=strict, raises=None
-        )
+        self.add_marker(pytest.mark.xfail(reason=msg))
 
     def addSuccess(self, testcase: "unittest.TestCase") -> None:
         pass