[Task] (BQ Java): Create an integration test that involves the use of both single and multi partition writes

### What needs to happen?

Need an integration test that fully makes use of [this flatten step in batch loads](https://github.com/ahmedabu98/beam/blob/ef85e6b948c4fd02b85f93e493e3dc50be2299c9/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/bigquery/BatchLoads.java#L409-L412).

Should look like a file loads write with dynamic destinations, one destination needing copy jobs, one needing only single load job.

### Issue Priority

Priority: 2

### Issue Component

Component: io-java-gcp

Start with the flatten step in sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/bigquery/BatchLoads.java at the linked lines, then locate the BigQuery Java integration-test entry point for file-load writes. Add coverage using dynamic destinations where one destination requires copy jobs and another requires only a single load job; the test should exercise both single- and multi-partition writes.

Written by the indexing model from the issue text.