cppcheck: use file list, add -j option, and handle build dir

[mdeweerd]

Why

In CI, I enabled cppcheck caching by adding the --cppcheck-build-dir option, but that did not work as every file was analysed in a separate cppcheck call.

The changes avoids splitting the analysis in multiple batches, and has all the files analysed in a single cppcheck call (which itself creates multiple subprocesses).

Summary of the changes

• Use --file-list option to pass multiple files to cppcheck in a single invocation
• Automatically add -j option with number of available processors for parallel analysis
• Add --cppcheck-build-dir when using -j > 1 to enable unusedFunction check compatibility
• Always suppress unusedFunction warnings (now safe due to --cppcheck-build-dir handling)
• Use truly unique temporary directories with tempfile.mkdtemp()
• Properly clean up temporary directories with shutil.rmtree() even on interruption

Changes

• Modified hooks/cppcheck.py to use --file-list option instead of running cppcheck once per file
• Added automatic -j N option with CPU count (only on multi-core systems)
• Added automatic --cppcheck-build-dir creation when using -j > 1
• Simplified suppression logic: always add --suppress=unusedFunction (previously conditional)
• Added proper cleanup of temporary build directories using shutil.rmtree()
• Updated test filters in tests/test_hooks.py to handle cppcheck warning messages

Extra Info

--suppress=unusedFunction is no longer needed

As we add the --cppcheck-build-dir when missing if -j is > 1, there is no longer an issue with the analysis.

Verification

• All existing tests pass - except on macos which is unrelated to these changes
• New behavior properly handles parallel analysis
• Temporary directories are cleaned up even on interruption
• No warning messages about -j and unusedFunction conflicts
• Test on actual project.