tests: run every test against a temp copy

[dingf3ng]

Why

coqpyt writes every accepted change straight back to the file on disk (coqpyt/coq/base_file.py::_make_change), and nothing puts it back. Any test pointed at a tracked examples/*.v therefore edits the repo.

CoqInterface.load() alone is enough — it pops the trailing Admitted.:

before:      4a0ceb92   tail: 'Proof.\n\nAdmitted.\n'
after load:  c0a19927   tail: 'Proof.\n'
after close: c0a19927   <- close() does NOT restore
git status:  M examples/main_loop_invariant_2_established_Coq.v

219dbad moved three tests onto temp_example_copy() and left ~12 behind. This finishes it.

The reset/restore pattern wasn't safe either

Some tests guarded themselves with reset_coq_file_to_admitted(...) + a finally: restore. A finally: block only runs while the interpreter unwinds:

signal	finally: runs?
SIGTERM	NO
SIGINT	yes
SIGKILL	NO

SIGTERM is what plain timeout sends, what docker stop sends, and what CI sends on a step timeout. These tests drive coq-lsp, so being killed is how they routinely end.

Reproducer — nothing exotic:

timeout -s TERM 6 python3 tests/test_coqpyt_simple.py

leaves examples/example.v holding Admitted. in place of its proof, plus an orphan example.v.backup that .gitignore hides from git status.

What changed

• 13 remaining tests moved to temp_example_copy()
• temp_example_copy() also copies examples/_CoqProject beside the copy, so tests driving coqpyt's ProofFile directly still resolve their libframac imports
• two follow-ons in the same files: test_proof_tree_step_by_step wrote PNGs into examples/; test_coqpyt_svcomp_clean printed "file not available" and carried on — now a real pytest.skip