Allow stdin fuzz targets to suppress logs

[joostjager]

Add an environment-variable switch that lets stdin fuzz targets use the dev-null test logger. This keeps direct invocations verbose by default, while external runners can opt into quieter passing-case replays.

I use stdin_fuzz also to run big sets of test cases using a runner that parallelizes and also continues on failure to examine the rest. Cargo test and test_cases dir approach do not work for that. The logging is a big slow down.

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[ldk-claude-review-bot]

Good - the types are compatible. DevNull and Stdout both implement Output + Clone + 'static, and the function accepts Out: test_logger::Output.

This PR is a clean, mechanical change. Every file follows the identical pattern, the template is updated, and no fuzz targets were missed. The DevNull type already existed and is properly implemented.

No issues found.