Open streams_test_tmp file in temporary folder

The tests `streams_tests/streams_buffered_file` and `streams_tests/streams_buffered_file_rand`
did not use a the temporary directory provided by  `BasicTestingSetup`, so it was not possible
to execute multiple of them in parallel. This fixes that.

To reproduce, run

```sh
parallel --halt now,fail=1 './src/test/test_bitcoin --run_test=streams_tests/streams_buffered_file_rand' -- ::: {1..1000}
```

This executes the test 1000 times, one job per CPU. It works on that commit but mergebase fails quickly.

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