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

Fixes https://github.com/pytest-dev/pytest/issues/5991
Fixes https://github.com/pytest-dev/pytest/issues/3823

Ref: https://github.com/pytest-dev/pytest-django/issues/772
Ref: https://github.com/pytest-dev/pytest/pull/1890
Ref: https://github.com/pytest-dev/pytest-django/pull/782

- inject wrapped testMethod

- adjust test_trial_error

- add test for `--trace` with unittests

doc/en/unittest.rst

@@ -238,17 +238,6 @@ was executed ahead of the ``test_method``.
 
 .. _pdb-unittest-note:
 
-.. note::
-
-    Running tests from ``unittest.TestCase`` subclasses with ``--pdb`` will
-    disable tearDown and cleanup methods for the case that an Exception
-    occurs. This allows proper post mortem debugging for all applications
-    which have significant logic in their tearDown machinery. However,
-    supporting this feature has the following side effect: If people
-    overwrite ``unittest.TestCase`` ``__call__`` or ``run``, they need to
-    to overwrite ``debug`` in the same way  (this is also true for standard
-    unittest).
-
 .. note::
 
     Due to architectural differences between the two frameworks, setup and