Rewrite fcnt14 test

This test has been changed in order to generate testcases at runtime.

The new algorithm sets a random starting position on file using
lseek(), it randomly generates fcntl() parameters for parent and
child and it verifies that fcntl() will raise a blocking error on
child when it's supposed to.

Reviewed-by: Cyril Hrubis <[email protected]>
Signed-off-by: Andrea Cervesato <[email protected]>