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GH-44337: [CI][GLib] Fix a flaky StreamDecoder and Buffer test (#44341)

### Rationale for this change

It's related to GC.

StreamDecoder accepts incomplete data. They are kept until enough data are provided. A caller must not release the incomplete data before they are processed. If they are released, StreamDecoder may touch unexpected data.

### What changes are included in this PR?

Refer unprocessed data until they are processed.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* GitHub Issue: #44337

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>