Run different command for system_benchmark package (logstash)

[mrodm]

Ensure that elastic-package benchmark system command is run for system_benchmark test package (under with-logstash folder).

Currently, for this test package is run the elastic-package test command:
https://buildkite.com/elastic/elastic-package/builds/2823#018ee67c-5012-4841-b76a-9343105e1fe1/80-633

+ for d in test/packages/${PACKAGE_TEST_TYPE:-other}/${PACKAGE_UNDER_TEST:-*}/ + cd test/packages/with-logstash/system_benchmark/ + '[' with-logstash == benchmarks ']' + '[' with-logstash == with-logstash ']' + '[' '*' == system_benchmark ']' + elastic-package test -v --report-format xUnit --report-output file --defer-cleanup 1s --test-coverage --coverage-format=generic 2024/04/16 10:46:15 DEBUG Enable verbose logging 

With this PR, it executes the elastic-package benchmark system :
https://buildkite.com/elastic/elastic-package/builds/2834#018ee744-75f6-460b-8ced-b864e8ff9a6b/111-660

+ cd test/packages/with-logstash/system_benchmark/ ++ basename test/packages/with-logstash/system_benchmark/ + package_to_test=system_benchmark + '[' with-logstash == benchmarks ']' + '[' with-logstash == with-logstash ']' + '[' system_benchmark == system_benchmark ']' + elastic-package benchmark system --benchmark logs-benchmark -v --defer-cleanup 1s 

Relates #1615

[jsoriano]

👍

[bhapas]

lgtm