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

topen.py

@@ -132,7 +132,7 @@ class Opt:
     metavar: str | None = None
     cast: type | Callable = str
     help_text: str = ""
-    flag: bool = False
+    is_flag: bool = False
 
 
 def _real_path(p: Path | str) -> Path:
@@ -224,7 +224,8 @@ OPTIONS: dict[str, Opt] = {
         "notes.quiet",
         default=False,
         cast=_strtobool,
-        help_text="Silence any verbose displayed information",
+        help_text="Silence any verbosely displayed information",
+        is_flag=True,
     ),
 }
 
@@ -321,6 +322,15 @@ you view the task.
     for key, opt in OPTIONS.items():
         if opt.cli is None:
             continue
+        if opt.is_flag:
+            parser.add_argument(
+                *opt.cli,
+                dest=key,
+                help=opt.help_text,
+                default=None,
+                action="store_true",
+            )
+            continue
         parser.add_argument(
             *opt.cli,
             dest=key,