Changes

Use Sys.command instead of Unix.system to call 'cmp'

Clang's thread sanitizer TSAN reports a (false?) alarm on Unix.system
in multithreaded programs now that Unix.system is implemented
on top of posix_spawn.

This commit replaces Unix.system with Sys.command, which might work
better with TSAN, based on preliminary experiments.

Note that this test is about file I/O, not Unix.system, so the replacement
is acceptable.  In parallel, we need to understand what's happening
with TSAN and posix_spawn.
 (commit: 89beb4c)