test: add channel filter and JSON log tests

TestChannelFilter: allowed/denied/PM/no-config/core-exempt/ampersand.
TestChannelConfig: TOML loading, defaults. TestJsonFormatter: fields,
exception, unicode, single-line, timestamp format.

tests/test_log.py

@@ -0,0 +1,77 @@
+"""Tests for structured JSON logging."""
+
+import json
+import logging
+
+from derp.log import JsonFormatter
+
+
+class TestJsonFormatter:
+    """Test JsonFormatter output."""
+
+    @staticmethod
+    def _make_record(msg: str, level: int = logging.INFO,
+                     name: str = "derp.test") -> logging.LogRecord:
+        """Create a LogRecord for testing."""
+        return logging.LogRecord(
+            name=name, level=level, pathname="", lineno=0,
+            msg=msg, args=(), exc_info=None,
+        )
+
+    def test_basic_fields(self):
+        fmt = JsonFormatter()
+        record = self._make_record("hello world")
+        output = fmt.format(record)
+        entry = json.loads(output)
+        assert entry["msg"] == "hello world"
+        assert entry["level"] == "info"
+        assert entry["logger"] == "derp.test"
+        assert "ts" in entry
+
+    def test_level_name(self):
+        fmt = JsonFormatter()
+        record = self._make_record("warn", level=logging.WARNING)
+        entry = json.loads(fmt.format(record))
+        assert entry["level"] == "warning"
+
+    def test_no_exc_key_without_exception(self):
+        fmt = JsonFormatter()
+        record = self._make_record("clean")
+        entry = json.loads(fmt.format(record))
+        assert "exc" not in entry
+
+    def test_exception_included(self):
+        fmt = JsonFormatter()
+        import sys
+        try:
+            raise ValueError("boom")
+        except ValueError:
+            exc_info = sys.exc_info()
+            record = logging.LogRecord(
+                name="derp.test", level=logging.ERROR, pathname="", lineno=0,
+                msg="failed", args=(), exc_info=exc_info,
+            )
+        entry = json.loads(fmt.format(record))
+        assert "exc" in entry
+        assert "ValueError" in entry["exc"]
+        assert "boom" in entry["exc"]
+
+    def test_output_is_single_line(self):
+        fmt = JsonFormatter()
+        record = self._make_record("one liner")
+        output = fmt.format(record)
+        assert "\n" not in output
+
+    def test_unicode_preserved(self):
+        fmt = JsonFormatter()
+        record = self._make_record("cafe\u0301")
+        entry = json.loads(fmt.format(record))
+        assert entry["msg"] == "cafe\u0301"
+
+    def test_timestamp_format(self):
+        fmt = JsonFormatter()
+        record = self._make_record("ts check")
+        entry = json.loads(fmt.format(record))
+        # ISO-ish: YYYY-MM-DDTHH:MM:SS
+        assert "T" in entry["ts"]
+        assert len(entry["ts"]) == 19