fix StreamStringTermsAggregatorTests by replacing RandomIndexWriter with plain IndexWriter opensearch-project#18956 WIP (opensearch-project#19404)

* fix opensearch-project#18956 WIP

- I replaced RandomIndexWriter to plain IndexWriter to avoid unexpected segments in test which are unable to handle more than single segment
- for other test I've added explicit segmenting. TODO It needs to be reviewed.
- Perhaps StreamingTermsAggregator should reject second segment via assert or there should be a proper publish/reset flow. TODO, WIP

so far this commit fixes the known reason for flakiness (I believe)

Signed-off-by: Mikhail Khludnev <[email protected]>

* newIndexWriterConfig() causes segmenting via flushes by bufferedDocs

Signed-off-by: Mikhail Khludnev <[email protected]>

* subAggs test goes single segment

Signed-off-by: Mikhail Khludnev <[email protected]>

* asserting single segment

Signed-off-by: Mikhail Khludnev <[email protected]>

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Signed-off-by: Mikhail Khludnev <[email protected]>

Author: mkhludnev