selftests: Fix detection of nettest command in fcnal-test

Most of the tests run by fcnal-test.sh relies on the nettest command.
Rather than trying to cover all of the individual tests, check for the
binary only at the beginning.

Also removes the need for log_error which is undefined.

Fixes: 6f9d5cac ("selftests: Setup for functional tests for fib and socket lookups")
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>