stestr run specific test but it is skipped by other test's skiptest requirement

[onstring]

Issue description

Using stestr run -n TEST_ID to run a specific test case, it is skipped by other test cases @testtools.skipUnless statements (my test does not have this skipUnless statements). But If I use stestr run TEST_ID, it will work as expected.

To be specific, I was trying to run openstack tempest ec2api tests, this test_create_delete_instance case does not have any skip test requirement.

>> stestr run --no-discover ec2api_tempest_plugin.api.test_instances.InstanceTest.test_create_delete_instance                                                                          
Traceback (most recent call last):
  File "/home/xxxxx/.pyenv/versions/3.8.10/lib/python3.8/runpy.py", line 194, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/home/xxxxx/.pyenv/versions/3.8.10/lib/python3.8/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/home/xxxxx/.pyenv/versions/3.8.10/envs/routine3/lib/python3.8/site-packages/stestr/subunit_runner/run.py", line 93, in <module>
    main()
  File "/home/xxxxx/.pyenv/versions/3.8.10/envs/routine3/lib/python3.8/site-packages/stestr/subunit_runner/run.py", line 87, in main
    program.TestProgram(
  File "/home/xxxxx/.pyenv/versions/3.8.10/envs/routine3/lib/python3.8/site-packages/stestr/subunit_runner/program.py", line 169, in __init__
    self.parseArgs(argv)
  File "/home/xxxxx/.pyenv/versions/3.8.10/lib/python3.8/unittest/main.py", line 147, in parseArgs
    self.createTests()
  File "/home/xxxxx/.pyenv/versions/3.8.10/lib/python3.8/unittest/main.py", line 158, in createTests
    self.test = self.testLoader.loadTestsFromNames(self.testNames,
  File "/home/xxxxx/.pyenv/versions/3.8.10/lib/python3.8/unittest/loader.py", line 220, in loadTestsFromNames
    suites = [self.loadTestsFromName(name, module) for name in names]
  File "/home/xxxxx/.pyenv/versions/3.8.10/lib/python3.8/unittest/loader.py", line 220, in <listcomp>
    suites = [self.loadTestsFromName(name, module) for name in names]
  File "/home/xxxxx/.pyenv/versions/3.8.10/lib/python3.8/unittest/loader.py", line 154, in loadTestsFromName
    module = __import__(module_name)
  File "/home/xxxxx/Repo/ec2api-tempest-plugin/ec2api_tempest_plugin/api/test_instances.py", line 28, in <module>
    class InstanceTest(base.EC2TestCase):
  File "/home/xxxxx/Repo/ec2api-tempest-plugin/ec2api_tempest_plugin/api/test_instances.py", line 198, in InstanceTest
    @testtools.skipUnless(CONF.aws.ebs_image_id, "EBS image id is not defined")
  File "/home/xxxxx/Repo/ec2api-tempest-plugin/ec2api_tempest_plugin/config.py", line 84, in __getattr__
    self._config = ConfigPrivate()
  File "/home/xxxxx/Repo/ec2api-tempest-plugin/ec2api_tempest_plugin/config.py", line 58, in __init__
    raise Exception('Config could not be found')
Exception: Config could not be found 

System information

• OS: Ubuntu 20.04
• stestr version (stestr --version): 3.2.0
• Python release (python --version): 3.8.10