doc/test-writing-guidelines: Add FILE and fork()

Add a paragraph about flushing FILE streams before doing fork() we
already talk about this problem briefly in 2.2.7 Fork()-ing but only for
the purpose of avoiding duplicate messages for tst_res().

Also change tst_fork() -> SAFE_FORK() which was missed when
documentation was updated for newlib.

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
1 file changed