test: do not start tests in parallel

Running the tests in parallel has two drawbacks:
- the tests are racing on the hugepages allocations,
- the tests are sharing the cores to run their checks which results in
  undeterministic execution time,

This results in random failures.
For better reproducibility in CI, start them all in a serialised way.

Signed-off-by: David Marchand <david.marchand@redhat.com>