Fix valgrind harness for expected-failure tests
Failure-path tests were masking valgrind leak reports because
--error-exitcode=42 was being treated as the expected non-zero
exit code. Add an expect_failure flag so these tests run valgrind
with --leak-check=no, while still checking ERROR SUMMARY for real
memory errors. Populate CLI output from stdout/stderr so valgrind
output is visible to postfix_check, and only print the captured
test output when a test fails.
closes https://github.com/official-stockfish/Stockfish/pull/7081
No functional change