cli: Ensure tests don't mess with the global logging setup

Notably, using logging.config.dictConfig disables all loggers before
setting its own config. There's no simple way of undoing these changes
to the logging config.

Even if we had a way to reset a basic logging config, pytest reuses its
log handler instances across tests, so it's not even that simple to be
able to reset the logging config from scratch and still have pytest able
to capture logs.

Instead of any of that, just add a fixture to reset the root logger
level, and to mock logging.config.dictConfig so that it doesn't actually
get called.

swh/core/tests/test_cli.py

@@ -12,6 +12,7 @@ import click
 from click.testing import CliRunner
 import pkg_resources
 import pytest
+import yaml
 
 help_msg_snippets = (
     (
@@ -70,6 +71,18 @@ def assert_section_contains(cli_output: str, section: str, snippet: str) -> bool
         )
 
 
+@pytest.fixture(autouse=True)
+def reset_root_loglevel():
+    root_level = logging.root.level
+    yield
+    logging.root.setLevel(root_level)
+
+
+@pytest.fixture
+def patched_dictconfig(mocker):
+    yield mocker.patch("logging.config.dictConfig", autospec=True)
+
+
 def test_swh_help(swhmain):
     runner = CliRunner()
     result = runner.invoke(swhmain, ["-h"])
@@ -262,14 +275,17 @@ def log_config_path(tmp_path):
     yield str(tmp_path / "log_config.yml")
 
 
-def test_log_config(log_config_path, swhmain):
+def test_log_config(log_config_path, swhmain, patched_dictconfig):
+    """Check that --log-config properly calls :func:`logging.config.dictConfig`."""
+
     @swhmain.command(name="test")
     @click.pass_context
     def swhtest(ctx):
-        logging.debug("Root log debug")
-        logging.info("Root log info")
-        logging.getLogger("dontshowdebug").debug("Not shown")
-        logging.getLogger("dontshowdebug").info("Shown")
+        patched_dictconfig.assert_called_once_with(
+            yaml.safe_load(open(log_config_path, "r"))
+        )
+
+        click.echo("Hello SWH!")
 
     runner = CliRunner()
     result = runner.invoke(
@@ -282,23 +298,24 @@ def test_log_config(log_config_path, swhmain):
     )
 
     assert result.exit_code == 0
-    assert result.output.strip() == "\n".join(
-        [
-            "custom format:root:DEBUG:Root log debug",
-            "custom format:root:INFO:Root log info",
-            "custom format:dontshowdebug:INFO:Shown",
-        ]
-    )
+    assert result.output.strip() == "Hello SWH!"
+
 
+def test_log_config_log_level_interaction(log_config_path, swhmain, patched_dictconfig):
+    """Check that --log-config and --log-level work properly together, by calling
+    :func:`logging.config.dictConfig` then setting the loglevel of the root logger."""
 
-def test_log_config_log_level_interaction(log_config_path, swhmain):
     @swhmain.command(name="test")
     @click.pass_context
     def swhtest(ctx):
-        logging.debug("Root log debug")
-        logging.info("Root log info")
-        logging.getLogger("dontshowdebug").debug("Not shown")
-        logging.getLogger("dontshowdebug").info("Shown")
+        assert logging.root.level == logging.DEBUG
+        patched_dictconfig.assert_called_once_with(
+            yaml.safe_load(open(log_config_path, "r"))
+        )
+
+        click.echo("Hello SWH!")
+
+    assert logging.root.level != logging.DEBUG
 
     runner = CliRunner()
     result = runner.invoke(
@@ -307,18 +324,13 @@ def test_log_config_log_level_interaction(log_config_path, swhmain):
             "--log-config",
             log_config_path,
             "--log-level",
-            "INFO",
+            "DEBUG",
             "test",
         ],
     )
 
     assert result.exit_code == 0
-    assert result.output.strip() == "\n".join(
-        [
-            "custom format:root:INFO:Root log info",
-            "custom format:dontshowdebug:INFO:Shown",
-        ]
-    )
+    assert result.output.strip() == "Hello SWH!"
 
 
 def test_multiple_log_level_behavior(swhmain):