tests/conftest: pytest top-level configuration, mostly Fixtures

* `eddn_message()` fixture, set up to return a method when used, such that
  *that* can be called with a key to look up the approproiate test message.
* `test_messages` dictionary to support that.

src/tests/conftest.py

@@ -0,0 +1,32 @@
+"""General pytest configuration, including fixtures."""
+from typing import Callable, Optional
+
+import pytest
+
+"""A dictionary of test messages, all in string form."""
+test_messages = {
+    'plain_journal_scan_valid': '''{
+        "$schemaRef": "https://eddn.edcd.io/schemas/journal/1",
+        "header": {
+            "uploaderID": "valid journal message",
+            "softwareName": "pytest:Gateway.parse_and_error_handle",
+            "softwareVersion": "v0.0.1"
+        },
+        "message": {
+            "timestamp":"2021-11-05T15:46:28Z",
+            "event":"Scan",
+            "StarSystem":"Elphin",
+            "StarPos":[-30.12500,8.18750,-17.00000],
+            "SystemAddress":3932076118738
+        }
+    }'''
+}
+
+
+@pytest.fixture
+def eddn_message() -> Callable:
+    """Supply the requested test message."""
+    def _method(msg_type: str) -> Optional[str]:
+        return test_messages.get(msg_type)
+
+    return _method