tests: add CmdResult::stdout_is_fixture_bytes()
tests: add CmdResult::stdout_is_fixture_bytes()

Add helper method `CmdResult::stdout_is_fixture_bytes()`, which is
like `stdout_is_fixture()` but compares stdout to the raw bytes of a
given file instead of decoding the contents of the file to a UTF-8
string.

uutils/coreutils,uutils/coreutils,uutils/coreutils