Changes made in response to testing with the k8s process manager

[bieryAtFnal]

Description

In DUNE-DAQ/drunc#597, Claudia mentioned a couple of changes in this repo that would fix some errors that were seen when using the k8s process manager in testing on np04-srv-019.

This PR has those changes:

• the use of only lower-case letters when creating a random string that is included in the DAQ session name in the parts of the example_system_test that get run at EHN1
• the removal of the check on the DAQ session run time from the small_footprint_quick_test since that check does not produce consistent results for different process manager types

With these changes, the following steps should result in successful running of tests on np04-srv-019 with a recent nightly build:

export KUBECONFIG=/nfs/home/np04daq/.kube/config.019
dunedaq_integtest_bundle.sh --verb 2 -k 'min|small|example' --pytest-options "--process-manager-type k8s"

These changes are intended for fddaq-v5.8.0.

Type of change

• Optimization (non-breaking change that improves code/performance)

Testing checklist

• Full set of integration tests pass (dunedaq_integtest_bundle.sh)