general: move most core tests inside my.core.tests package

- distributes tests alongside the package, might be convenient for package users
- removes some weird indirection (e.g. dummy test files improting tests from modules)
- makes the command line for tests cleaner (e.g. no need to remember to manually add files to tox.ini)
- tests automatically covered by mypy (so makes mypy runs cleaner and ultimately better coverage)

The (vague) convention is

- tests/somemodule.py -- testing my.core.somemodule, contains tests directly re
- tests/test_something.py -- testing a specific feature, e.g. test_get_files.py tests get_files methon only

my/core/tests/test_tmp_config.py

@@ -0,0 +1,29 @@
+from ..cfg import tmp_config
+
+
+def _init_default_config() -> None:
+    import my.config
+
+    class default_config:
+        count = 5
+
+    my.config.simple = default_config  # type: ignore[assignment,misc]
+
+
+def test_tmp_config() -> None:
+    ## ugh. ideally this would be on the top level (would be a better test)
+    ## but pytest imports eveything first, executes hooks, and some reset_modules() fictures mess stuff up
+    ## later would be nice to be a bit more careful about them
+    _init_default_config()
+    from my.simple import items
+
+    assert len(list(items())) == 5
+
+    class config:
+        class simple:
+            count = 3
+
+    with tmp_config(modules='my.simple', config=config):
+        assert len(list(items())) == 3
+
+    assert len(list(items())) == 5