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/conftest.py

@@ -0,0 +1,27 @@
+from pathlib import Path
+
+import pytest
+
+from topen import OPTIONS
+
+
+@pytest.fixture
+def fake_id(monkeypatch):
+    monkeypatch.setattr("sys.argv", ["topen", "0"])
+
+
+@pytest.fixture
+def isolate_env(monkeypatch):
+    # delete all existing env vars that could interfere
+    monkeypatch.delenv("EDITOR", raising=False)
+    monkeypatch.delenv("VISUAL", raising=False)
+    for opt in OPTIONS.values():
+        if opt.env:
+            monkeypatch.delenv(opt.env, raising=False)
+
+
+@pytest.fixture
+def fake_rc(tmp_path: Path, monkeypatch):
+    rc = tmp_path / "test.taskrc"
+    monkeypatch.setattr(OPTIONS["task_rc"], "default", rc)
+    return rc