--files carries the whole list as one argv element, and Linux caps a
single argument at MAX_ARG_STRLEN (128 KiB) - a much smaller limit than
ARG_MAX. The whole-suite list (~210 KB) dies with E2BIG in execve before
the runner's first line runs, so 'run the whole suite except one file'
cannot be expressed through --files at all.
Add --files-from PATH (or '-' for stdin), one path per line, mutually
exclusive with --files. A bare '-' after --files-from is normalized to
the '='-joined form because argparse treats '-' as a positional.
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