rewrite functional backend test code in python

tests/functional/README.md

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+Tests for the functional backend use pytest as a testrunner.
+
+Run with `pytest -v`
+
+Pytest options you might want:
+
+- `-v`: More progress indication.
+
+- `--basetemp tmp`: Store test files (including vcd results) in tmp. 
+  CAREFUL: contents of tmp will be deleted
+
+- `-k <pattern>`: Run only tests that contain the pattern, e.g.
+  `-k cxx` or `-k smt` or `-k demux` or `-k 'cxx[demux`
+ 
+- `-s`: Don't hide stdout/stderr from the test code.
+
+Custom options for functional backend tests:
+
+- `--per-cell N`: Run only N tests for each cell.