test_exec: SO_KEEPALIVE value only needs to be true

test_exec: SO_KEEPALIVE value only needs to be true

On FreeBSD 10.3, the value of SO_KEEPALIVE returned by
getsockopt is 8, even when set to '1' via setsockopt.
Relax the test to only ensure the boolean value is
interpreted as "true".

Verified independently of Ruby using the following:
--------8<---------
    #include <sys/types.h>
    #include <sys/socket.h>
    #include <stdio.h>

static int err(const char *msg)
{
	perror(msg);
	return 1;
}

int main(void)
{
	int sv[2];
	int set = 1;
	int got;
	socklen_t len = (socklen_t)sizeof(int);
	int rc;

	rc = socketpair(PF_LOCAL, SOCK_STREAM, 0, sv);
	if (rc) return err("socketpair failed");

	rc = setsockopt(sv[0], SOL_SOCKET, SO_KEEPALIVE, &set, len);
	if (rc) return err("setsockopt failed");

	rc = getsockopt(sv[0], SOL_SOCKET, SO_KEEPALIVE, &got, &len);
	if (rc) return err("getsockopt failed");

	printf("got: %d\n", got);
	return 0;
}