add an 'untested' case when none of the atomic operations are available

- this fixes a build failure introduced in change 4913 when
  compiling with ATF and --disable-atomic
7 jobs for 154-fix-atomic-test in 6 minutes and 30 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Precheck
passed #4682
linux docker
precheck:debian:sid:amd64

00:00:10

 
  Build
passed #4683
linux docker
build:debian:sid:amd64

00:01:33

passed #4684
linux docker
build:debian:sid:i386

00:01:39

 
  Test
passed #4687
docker linux
systemtest:debian:sid:amd64

00:04:38

passed #4688
docker linux
systemtest:debian:sid:i386

00:04:39

passed #4685
docker linux
unittest:debian:sid:amd64

00:00:45

passed #4686
docker linux
unittest:debian:sid:i386

00:00:47