selftests/seccomp: More closely track fds being assigned

Since the open fds might not always start at "4" (especially when
running under kselftest, etc), start counting from the first assigned
fd, rather than using the more permissive EXPECT_GE(fd, 0).

Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/lkml/20210527032948.3730953-1-keescook@chromium.org
Reviewed-by: Rodrigo Campos <rodrigo@kinvolk.io>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>