test: Restructure test files

Extract the individual parsing tests (cli, env, rc) and add additional
configuration test file (TConf and config builder).

To extract the fixtures they have to go into an additional 'conftest.py'
file for pytest to recognize and automatically import them, see:
https://docs.pytest.org/en/stable/reference/fixtures.html#conftest-py-sharing-fixtures-across-multiple-files

and:
https://docs.pytest.org/en/stable/how-to/fixtures.html#using-fixtures-from-other-projects

test/test_parse_cli.py

@@ -0,0 +1,37 @@
+from pathlib import Path
+
+from topen import parse_cli
+
+
+class TestCli:
+    def test_cli_minimum_id(self, monkeypatch):
+        monkeypatch.setattr("sys.argv", ["topen", "42"])
+        assert parse_cli() == {"task_id": "42"}
+
+    def test_cli_options(self, monkeypatch):
+        monkeypatch.setattr(
+            "sys.argv",
+            [
+                "topen",
+                "123",
+                "--extension",
+                "txt",
+                "--editor",
+                "vim",
+                "--annotation",
+                "HERENOTE",
+            ],
+        )
+        assert parse_cli() == {
+            "task_id": "123",
+            "notes_ext": "txt",
+            "notes_editor": "vim",
+            "notes_annot": "HERENOTE",
+        }
+
+    def test_cli_parses_paths(self, monkeypatch):
+        monkeypatch.setattr(
+            "sys.argv",
+            ["topen", "123", "--notes-dir", "/somewhere/else"],
+        )
+        assert parse_cli()["notes_dir"] == Path("/somewhere/else")