in_systemd: tests: Provide restoring way the previous behavior

[cosmo0920]

The revised behavior needs more memory than just serializing one-by-one way.
So, for the lower memory environments or resource capped containers, we need to provide a restoring way with previous behavior.
With the previous behavior, the last element on the duplicated key is only effective when encoding as JSON.

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Testing
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[edsiper]

@cosmo0920 are we sure is not a memory leak ? I ask since the user reported excessive memory usage:

https://fluent-all.slack.com/archives/C0CTQGHKJ/p1734955974377129

[cosmo0920]

@cosmo0920 are we sure is not a memory leak ? I ask since the user reported excessive memory usage:

https://fluent-all.slack.com/archives/C0CTQGHKJ/p1734955974377129

Yeah, there is a possibility to be existing some of the memory leaks. But surely we increased memory usages in this plugin with using the mediate representation via cfl kvlist.