Call cleanup functions if explicit teardown is needed

Not calling cleanup functions after a test fails may affect other tests
down the row.

This issue was introduced in 04f27d4.

When a test fails, a `GetOutOf_testPartExecutor` exception is raised to
make `unittest` halt `testPartExecutor`:

https://github.com/pytest-dev/pytest/blob/413ca8a4d097ed1a98b2d1012ca7df17aa6837b1/src/_pytest/unittest.py#L206-L228

That prevents `teardown` **and** cleanup functions from being called.
That's why `self._needs_explicit_tearDown` is set to `True` to then
explicitly call `teardown` later on:

https://github.com/pytest-dev/pytest/blob/413ca8a4d097ed1a98b2d1012ca7df17aa6837b1/src/_pytest/unittest.py#L122-L126

But no cleanup functions are called. Hence this PR.

Author: scorphus

AUTHORS

@@ -211,6 +211,7 @@ Omar Kohl
 Omer Hadari
 Ondřej Súkup
 Oscar Benjamin
+Pablo Aguiar
 Patrick Hayes
 Pauli Virtanen
 Paweł Adamczak

changelog/6947.bugfix.rst

@@ -0,0 +1 @@
+Call cleanup functions on test failure.

src/_pytest/unittest.py

@@ -122,6 +122,7 @@ def setup(self):
     def teardown(self):
         if self._needs_explicit_tearDown:
             self._testcase.tearDown()
+            self._testcase.doCleanups()
         self._testcase = None
         self._obj = None
 

testing/test_unittest.py

@@ -876,6 +876,24 @@ def test_notTornDown():
     reprec.assertoutcome(passed=1, failed=1)
 
 
+def test_call_cleanup_functions_on_test_failure(testdir):
+    testdir.makepyfile(
+        """
+        import unittest
+        class TC(unittest.TestCase):
+            def setUp(self):
+                self.addCleanup(lambda: print("someCleanup()"))
+            def test_method(self):
+                assert False
+    """
+    )
+    result = testdir.runpytest("-s")
+    assert result.ret == 1
+    result.stdout.fnmatch_lines(
+        ["*someCleanup()*", "*test_method*", "*assert False*", "*1 failed*"]
+    )
+
+
 def test_issue333_result_clearing(testdir):
     testdir.makeconftest(
         """