tests: benchmarks/app_kernel: enable for floating point

The app_kernel benchmarking app has the config file for benchmarking
with floating point enabled, but it was never used. So add it
to the testcase.yaml.

Note that this also limits to run on one CPU on a SMP system as
the resulting numbers would be more consistent among runs.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>