[TEST] fix MultiMatchQueryIT random docs generation so that they don't interfere in score tests

When generating random bogus documents, it could happen that they contain both the terms "the" and "ultimate", which would match the query "the ultimate" better than all the other non bogus documents, which would cause testCrossFieldMode to fail. "the" is a term that's relatively likely to be randomly generated given its length; we can simply increase the minimum length of randomly generated terms to 5, so that there are no collisions, as "the" cannot be generated anymore (nor can "ultimate" as the lenght doesn't go up to 8).

Also made some assertions more accurate to check how many hits match a query rather than checking only that the first or second hits are there.

Closes #18873

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