Do not install tests in site-packages

[musicinmybrain]

Fixes:

python3.11 -m venv _e
. _e/bin/activate
pip install pymapvbvd
ls -l _e/lib/python3.11/site-packages/tests/

total 20
-rw-r--r--. 1 ben ben   39 Jul  8 17:53 __init__.py
drwxr-xr-x. 1 ben ben  254 Jul  8 17:53 __pycache__
-rw-r--r--. 1 ben ben 2436 Jul  8 17:53 test_flags.py
-rw-r--r--. 1 ben ben 2168 Jul  8 17:53 test_read.py
-rw-r--r--. 1 ben ben 1367 Jul  8 17:53 test_slicing.py
-rw-r--r--. 1 ben ben 2121 Jul  8 17:53 test_svs.py

That is, pymapvbvd is incorrectly installing its tests/ as a top-level tests package.