unittest: do not use TestCase.debug with --pdb

Reverts pytest-dev#1890, which needs to
be fixed/addressed in another way
(pytest-dev#5996).

Fixes pytest-dev#5991.

changelog/5991.bugfix.rst

@@ -0,0 +1 @@
+Do not use ``TestCase.debug()`` for unittests with ``--pdb``.

src/_pytest/unittest.py

@@ -203,13 +203,7 @@ def _handle_skip(self):
         return False
 
     def runtest(self):
-        if self.config.pluginmanager.get_plugin("pdbinvoke") is None:
-            self._testcase(result=self)
-        else:
-            # disables tearDown and cleanups for post mortem debugging (see #1890)
-            if self._handle_skip():
-                return
-            self._testcase.debug()
+        self._testcase(result=self)
 
     def _prunetraceback(self, excinfo):
         Function._prunetraceback(self, excinfo)

testing/test_pdb.py

@@ -160,6 +160,7 @@ def flush(child):
             child.wait()
         assert not child.isalive()
 
+    @pytest.mark.xfail(reason="running .debug() all the time is bad (#5991)")
     def test_pdb_unittest_postmortem(self, testdir):
         p1 = testdir.makepyfile(
             """
@@ -181,21 +182,37 @@ def test_false(self):
         self.flush(child)
 
     def test_pdb_unittest_skip(self, testdir):
-        """Test for issue #2137"""
+        """Test for issues #2137 and #5991"""
         p1 = testdir.makepyfile(
             """
             import unittest
+
             @unittest.skipIf(True, 'Skipping also with pdb active')
             class MyTestCase(unittest.TestCase):
                 def test_one(self):
                     assert 0
+
+            class MyOtherTestCase(unittest.TestCase):
+                def setUp(self):
+                    print("\\nsetUp_called\\n")
+
+                def tearDown(self):
+                    print("\\ntearDown_called\\n")
+
+                def test_two(self):
+                    self.skipTest("skip_two")
         """
         )
-        child = testdir.spawn_pytest("-rs --pdb %s" % p1)
-        child.expect("Skipping also with pdb active")
-        child.expect("1 skipped in")
-        child.sendeof()
-        self.flush(child)
+        result = testdir.runpytest("-s", "-rs", "--pdb", str(p1))
+        result.stdout.fnmatch_lines(
+            [
+                "setUp_called",
+                "tearDown_called",
+                "SKIPPED [1] test_pdb_unittest_skip.py:5: Skipping also with pdb active",
+                "SKIPPED [1] test_pdb_unittest_skip.py:15: skip_two",
+                "*= 2 skipped in *",
+            ]
+        )
 
     def test_pdb_print_captured_stdout_and_stderr(self, testdir):
         p1 = testdir.makepyfile(