test_unittest: TestBreakSignalDefault.testInstallHandler() fails if run after TestBreak:
---------
$ ./python -m test -v test_unittest -R 3:3 -m '*TestBreak.testHandlerReplacedButCalled' -m '*TestBreak.testInstallHandler' -m '*TestBreakSignalDefault.testInstallHandler'
(...)
beginning 6 repetitions
123456
testHandlerReplacedButCalled (unittest.test.test_break.TestBreak) ... ok
testInstallHandler (unittest.test.test_break.TestBreak) ... ok
testInstallHandler (unittest.test.test_break.TestBreakSignalDefault) ... ok

----------------------------------------------------------------------
Ran 3 tests in 0.003s

OK
(...)
.testHandlerReplacedButCalled (unittest.test.test_break.TestBreak) ... ok
testInstallHandler (unittest.test.test_break.TestBreak) ... ok
testInstallHandler (unittest.test.test_break.TestBreakSignalDefault) ... FAIL

======================================================================
FAIL: testInstallHandler (unittest.test.test_break.TestBreakSignalDefault)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/vstinner/python/main/Lib/unittest/test/test_break.py", line 38, in testInstallHandler
    self.assertTrue(unittest.signals._interrupt_handler.called)
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: False is not true
(...)
---------

Or you can use a "bisect" file:
---
unittest.test.test_break.TestBreak.testHandlerReplacedButCalled
unittest.test.test_break.TestBreak.testInstallHandler
unittest.test.test_break.TestBreakSignalDefault.testInstallHandler
---

And the command:

./python -m test -v -F test_unittest --matchfile=bisect

Or the command:

./python -m test -v test_unittest -R 3:3 --matchfile=bisect

It fails starting at the 4th iteration.