Snapshots: safer handling of missing snapshots #119

Part of #101. Line numbers refer to branch chore/php-8.4-dependabot (#98). Verified = reproduced, suspected = from reading the code.

A missing snapshot is created and the test passes, so in CI a snapshot someone forgot to commit gives a green test that asserts nothing. CREATE_SNAPSHOTS=0 exists, but has to be set explicitly.

• Opt-in: mark the test incomplete or risky when a snapshot was created.
• Opt-in: refuse to create snapshots when CI is set, unless creation is explicitly allowed.