test: move the ADR scanner's self-tests out of the file consumers fetch

[lesnik512]

What

tests/test_adr_citations.py, the file every repo fetches under CI9, keeps the scanner and the one invariant. Its eight tmp_path self-tests move to tests/test_adr_scanner.py, which imports the scanner from the fetched file. The three constants the self-tests use lose their leading underscore, since they are now imported across modules.

Nothing changes outside this repo: same URL, same recipe, same bytes-for-bytes rule in CI9.

Why

Consumers ran nine tests per repo of which one said anything about that repo. The self-tests exist to prove the scanner works; that proof belongs where the file is edited, and this repo still runs all nine on every change. The bytes a consumer runs are the bytes this repo's suite exercised.

Verified

• This repo: 134 passed, 29 skipped; the two files collect 1 + 8.
• Fetched file alone, --rootdir=. --noconftest -o addopts=, against eof-fixer and that-depends: 1 passed each; with a planted docs/adr/0009-nope.md citation: fails naming the file and the citation.
• ruff check --select F clean on both files.