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

Fixes pytest-dev#5991.

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

@@ -164,20 +164,31 @@ def test_pdb_unittest_postmortem(self, testdir):
         p1 = testdir.makepyfile(
             """
             import unittest
+
+            teardown_called = 0
+
             class Blub(unittest.TestCase):
                 def tearDown(self):
-                    self.filename = None
-                def test_false(self):
+                    global teardown_called
+                    teardown_called += 1
+
+                def test_error(self):
                     self.filename = 'debug' + '.me'
                     assert 0
+
+                def test_check(self):
+                    assert teardown_called == 1
         """
         )
-        child = testdir.spawn_pytest("--pdb %s" % p1)
+        child = testdir.spawn_pytest(
+            "--pdb {p1}::Blub::test_error {p1}::Blub::test_check".format(p1=p1)
+        )
         child.expect("Pdb")
         child.sendline("p self.filename")
-        child.sendeof()
+        child.expect("'debug.me'")
+        child.sendline("c")
         rest = child.read().decode("utf8")
-        assert "debug.me" in rest
+        assert "= 1 failed, 1 passed in" in rest
         self.flush(child)
 
     def test_pdb_unittest_skip(self, testdir):