tests: stat: cover %N escaping of control characters

tests: stat: cover %N escaping of control characters

The default shell-escape quoting style escapes control characters in
file names (e.g. newlines) as $'...' sequences, but no test exercised
this; only the single-quote character was covered.
referenced here: https://github.com/uutils/coreutils/issues/9925

* tests/stat/stat-fmt.sh: Add a case with a file name containing
contiguous and separated newlines.
https://github.com/coreutils/coreutils/pull/271