Fixed fgets_eagain unit test.

On a non-blocking pipe fgets sets EAGAIN when it encounters partial lines. No
error is set on full lines or EOF. EOF is reached when the writing end of the
pipe is closed. Partial lines and full lines are both returned by fgets, EOF
results in NULL.

Mention of this behaviour can be found in #1903 and #2045.
add changes file for 12503