SI-8924 No outer ref in LinearSeq iterator

The iterator for `LinearSeqLike` holds an outer pointer via
the call to `newBuilder`, which is used to empty the underlying
sequence on `toList`.

A better strategy is to use `these take 0` to create the empty
`Seq`.  We can rely on `take 0` to mean the empty `Repr`.

The unit test confirms that a weak reference to the sequence
is collectable after `seq.iterator.toList`.
I think you need to call `t.join()` to ensure that memory pressure thread is stopped before start of next unit test.