Add unittest suite for Sitzplatzverteilung with three scenarios

Covers three concrete seating layouts to exercise the GA's group
placement and splitting logic:
- three tables chained pairwise (2 seats each), three couples ->
  no group gets split, optimal score of 0.
- four tables in a line (2 seats each), two groups of four -> each
  group ends up split across one contiguous table pair (T1+T2 or
  T3+T4), never mixed across the middle seam.
- six tables in a circle (2 seats each), four groups of three ->
  each group is split exactly once (2+1), since no table has three
  seats; with the fixed seed the split pairs are also true circle
  neighbors.

Also documents how to run the suite in README.md, including the
PYTHONPATH workaround needed when a stray global env var shadows
the stdlib "tests" package name with an unrelated project's tests.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -16,6 +16,7 @@ platz/
 ├── bin/           # environment + launcher scripts (.bat + .sh pairs)
 ├── cfg/           # GA cycle definition and penalty scheme
 ├── libs/          # the two Python modules (ga.py, Strukturdaten.py, platz.py)
+├── tests/         # unittest suite for Strukturdaten.py
 └── work/          # per-run input (table layout, guest list) and output
     ├── test1/
     └── test2/
@@ -26,6 +27,16 @@ platz/
 - `libs/Strukturdaten.py` — domain model for the seating problem (`Person`, `Gruppe`,
   `Tisch`, `Strafliste`, `Sitzplatzverteilung`, `XMLConfig`).
 
+## Running the tests
+
+```bash
+python -m unittest discover -s tests -p "test_*.py" -v
+```
+
+If a stray global `PYTHONPATH` entry shadows the standard-library `tests` package name
+(unrelated to this project), clear it for the command, e.g. `PYTHONPATH= python -m
+unittest discover -s tests -p "test_*.py" -v`.
+
 ## Setup
 
 1. Install a Python interpreter (`py` on Windows / `python3` on Linux/macOS must be on