tests: Prevent invocation in root dir, point to src/

Because the code itself only runs properly from inside the `src/` directory,
you cannot run `pytest` from the root directory and have the tests work.

So, set things up such that running `pytest` in the root directory will
only run the one test, and have that test point out the user error.

pytest.ini

@@ -0,0 +1,3 @@
+[pytest]
+norecursedirs = src
+testpaths = tests/

tests/test_run_pytest_in_src_dir_not_here.py

@@ -0,0 +1,3 @@
+"""Inform the invoker they ran `pytest` in the wrong place."""
+print('You must run `pytest` from inside the `src/` directory')
+assert False