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/__init__.py

@@ -0,0 +1,3 @@
+# hmm, sadly pytest --import-mode importlib --pyargs my.core.tests doesn't work properly without __init__.py
+# although it works if you run either my.core or my.core.tests.sqlite (for example) directly
+# so if it gets in the way could get rid of this later?