[system/ntp] system tests: use local NTP server instead of pool.ntp.org

[mykola-elastic]

For system tests of system/ntp data stream: Use local NTP server instead of pool.ntp.org to remove flakiness.

Querying a public resource pool.ntp.org is flaky by definition.

Proposed commit message

See title.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.
• I have verified that any added dashboard complies with Kibana's Dashboard good practices

Related issues

• Closes [Subscription basic] [system] Failing test daily: system test: default in system.ntp #16708
• Closes [Stack 9.3.0-SNAPSHOT] [system] Failing test daily: system test: default in system.ntp #16633

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Package system - 2.9.1 containing this change is available at https://epr.elastic.co/package/system/2.9.1/