DAOS-16460 test: Improve get_service_file() (#15116)

[phender]

Handle unexpected output when parsing the systemctl service file.

Skip-unit-tests: true
Skip-fault-injection-test: true
Test-tag: HarnessCoverageTest

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[github-actions]

Ticket title is 'LRZ: three engine coredumps 2024-08-28'
Status is 'In Progress'
Labels: 'LRZ,scrubbed_2.8'
https://daosio.atlassian.net/browse/DAOS-16460

[phender]

Changes are working in https://build.hpdd.intel.com/job/daos-stack/job/daos/job/PR-15121/1/artifact/Functional%20on%20EL%208.8/launch/functional_on_el_8.8/job.log

2024/09/11 02:40:00 INFO        __add_systemctl_override: Setting up systemctl override for daos_server.service
2024/09/11 02:40:00 DEBUG                     run_remote: Running on wolf-240vm[2-9] with a 120 second timeout: systemctl show daos_server.service | grep 'FragmentPath='
2024/09/11 02:40:00 DEBUG                log_result_data:   wolf-240vm[2-9] (rc=0): FragmentPath=/usr/lib/systemd/system/daos_server.service
2024/09/11 02:40:00 DEBUG                     run_remote: Running on wolf-240vm[2-9] with a 120 second timeout: sudo -n mkdir -p /usr/lib/systemd/system/daos_server.service.d
2024/09/11 02:40:01 DEBUG                log_result_data:   wolf-240vm[2-9] (rc=0): <no output>