grep::test_muptiple_pattern_files_multiple_input_files_line_number inconsistent, fails sometimes

[jgarzik]

Test output during failure:

---- grep::test_muptiple_pattern_files_multiple_input_files_line_number stdout ----
thread 'grep::test_muptiple_pattern_files_multiple_input_files_line_number' panicked at plib/src/testing.rs:55:10:
failed to write to stdin: Os { code: 32, kind: BrokenPipe, message: "Broken pipe" }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    grep::test_muptiple_pattern_files_multiple_input_files_line_number

Re-running failed CI job usually resolves issue.

[jgarzik]

Similar failures:

---- grep::test_duplicate_input_files_count stdout ----
thread 'grep::test_duplicate_input_files_count' panicked at plib/src/testing.rs:55:10:
failed to write to stdin: Os { code: 32, kind: BrokenPipe, message: "Broken pipe" }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

---- grep::test_multiple_input_files_line_number stdout ----
thread 'grep::test_multiple_input_files_line_number' panicked at plib/src/testing.rs:55:10:
failed to write to stdin: Os { code: 32, kind: BrokenPipe, message: "Broken pipe" }

[jgarzik]

Believed fixed by #233