tests/cut: ignore stdin write error on fast-exit delimiter test

[sylvestre]

test_single_quote_pair_as_delimiter_is_invalid is intermittent on i686: the parent writes "a''b\n" to cut's stdin while cut validates args and exits immediately with "delimiter must be a single character". On a fast child exit, the kernel closes the pipe read-end before the parent's write() runs, so write() returns EPIPE and the pipe_in thread panics.

The race is inherent: draining stdin in cut before exiting on an arg error would hang on terminals and read unbounded data from long pipes, contradicting GNU behavior. Use the test framework's ignore_stdin_write_error() flag, which exists for exactly this "parent writes to child that exits fast" pattern.

[sylvestre]

hopefully fixes: https://github.com/uutils/coreutils/actions/runs/24315448741/job/70992481916?pr=11781

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tty/tty-eof (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/dd/no-allocate is now passing!
Congrats! The gnu test tests/printf/printf-surprise is now passing!