tests: Move to loading test messages from files

* Implement a new fixture `eddn_message_from_file()` for testing this.
  Eventually this will get renamed to `eddn_message()`.

* Files containing one message each go under `tests/eddn_message`, with
  sub-directories for `<schema>/<version>[/<event>]` and then the filename
  is the nature of that file in that context.

  Completely invalid files, i.e. not even valid JSON, can be placed in
  the top level of `tests/eddn_message`.

* Use this for the `parse_and_error_handle()` "valid journal/1 scan" test.

src/tests/conftest.py

@@ -1,5 +1,6 @@
 """General pytest configuration, including fixtures."""
 import os
+import pathlib
 import sys
 from typing import Callable, Optional
 
@@ -89,6 +90,17 @@ test_messages = {
 }
 
 
+@pytest.fixture()
+def eddn_message_from_file() -> Callable:
+    """Load and supply a test message from the on-disk collection."""
+    def _method(msg_type: str) -> Optional[str]:
+        path = pathlib.Path('tests/eddn_message/' + msg_type)
+        with open(path, 'r') as eddn_message:
+            return eddn_message.read()
+
+    return _method
+
+
 @pytest.fixture
 def eddn_message() -> Callable:
     """Supply the requested test message."""

src/tests/gateway/test_parse_and_error_handle.py

@@ -26,9 +26,9 @@ def test_fail_validation_no_softwarename(fix_sys_path, eddn_gateway, eddn_messag
     assert res.startswith("FAIL: Schema Validation: [<ValidationError: \"'softwareName' is a required property\">]")
 
 
-def test_valid_journal_scan(fix_sys_path, eddn_gateway, eddn_message: Callable) -> None:
+def test_valid_journal_scan(fix_sys_path, eddn_gateway, eddn_message_from_file: Callable) -> None:
     """Test a valid journal/1, `event == 'Scan'` message."""
-    msg = eddn_message('plain_journal_scan_valid')
+    msg = eddn_message_from_file('journal/1/scan/valid.json')
     res = eddn_gateway.parse_and_error_handle(msg.encode(encoding="utf-8"))
     assert res == "OK"