Use pytest as a test runner instead of unit test

'./test <controller> -s spec1 -s spec2' becomes:
'pytest --controller <controller> -k "spec1 or spec2"'

This uses pytest's test selection, which allows finer selection of which tests
to run (for example, it will allow running all tests but those requiring one
feature or combination of features).
It also allows running only a particular test (or set of test) by
filtering on their name or file name.

pytest also shows a much nicer output while testing (grouped by file,
percentage of tests run, manages the verbosity); and it captures all the output
and only shows it if the test fails, which makes --show-io irrelevant.

conftest.py

@@ -0,0 +1,94 @@
+import importlib
+import sys
+import unittest
+
+import pytest
+import _pytest.unittest
+
+from irctest.cases import _IrcTestCase, BaseClientTestCase, BaseServerTestCase
+from irctest.basecontrollers import BaseClientController, BaseServerController
+
+def pytest_addoption(parser):
+    """Called by pytest, registers CLI options passed to the pytest command."""
+    parser.addoption("--controller", help="Which module to use to run the tested software.")
+    parser.addoption('--openssl-bin', type=str, default='openssl',
+            help='The openssl binary to use')
+
+
+def pytest_configure(config):
+    """Called by pytest, after it parsed the command-line."""
+    module_name = config.getoption("controller")
+
+    if module_name is None:
+        pytest.exit("--controller is required.", 1)
+
+    try:
+        module = importlib.import_module(module_name)
+    except ImportError:
+        pytest.exit('Cannot import module {}'.format(module_name), 1)
+
+    controller_class = module.get_irctest_controller_class()
+    if issubclass(controller_class, BaseClientController):
+        from irctest import client_tests as module
+    elif issubclass(controller_class, BaseServerController):
+        from irctest import server_tests as module
+    else:
+        pytest.exit(
+            r'{}.Controller should be a subclass of '
+            r'irctest.basecontroller.Base{{Client,Server}}Controller'
+            .format(module_name),
+            1
+        )
+    _IrcTestCase.controllerClass = controller_class
+    _IrcTestCase.controllerClass.openssl_bin = config.getoption("openssl_bin")
+    _IrcTestCase.show_io = True  # TODO
+
+
+def pytest_collection_modifyitems(session, config, items):
+    """Called by pytest after finishing the test collection,
+    and before actually running the tests.
+
+    This function filters out client tests if running with a server controller,
+    and vice versa.
+    """
+
+    # First, check if we should run server tests or client tests
+    if issubclass(_IrcTestCase.controllerClass, BaseServerController):
+        server_tests = True
+    elif issubclass(_IrcTestCase.controllerClass, BaseClientController):
+        server_tests = False
+    else:
+        assert False, (
+            f"{_IrcTestCase.controllerClass} inherits neither "
+            f"BaseClientController or BaseServerController"
+        )
+
+    filtered_items = []
+
+    # Iterate over each of the test functions (they are pytest "Nodes")
+    for item in items:
+        # we only use unittest-style test function here
+        assert isinstance(item, _pytest.unittest.TestCaseFunction)
+
+        # unittest-style test functions have the node of UnitTest class as parent
+        assert isinstance(item.parent, _pytest.unittest.UnitTestCase)
+
+        # and that node references the UnitTest class
+        assert issubclass(item.parent.cls, unittest.TestCase)
+
+        # and in this project, TestCase classes all inherit either from BaseClientController
+        # or BaseServerController.
+        if issubclass(item.parent.cls, BaseServerTestCase):
+            if server_tests:
+                filtered_items.append(item)
+        elif issubclass(item.parent.cls, BaseClientTestCase):
+            if not server_tests:
+                filtered_items.append(item)
+        else:
+            assert False, (
+                f"{item}'s class inherits neither BaseServerTestCase "
+                "or BaseClientTestCase"
+            )
+
+    # Finally, rewrite in-place the list of tests pytest will run
+    items[:] = filtered_items