tests/fdgrowtable: perform the threaded test in a child process

The test needs to be performed in a new process that was forked with
RFCFDG flag. The will guarantee that the table will start to grow from 20
file descriptors, no matter what kyua(1) or a bare shell was doing before
executing this test. This should fix repetitive test runs from a shell
as well as failures with kyua(1) in some environments.