BigQueryIO test using FakeBigQueryServices

The BigQueryIO can be tested using various FakeServices like
* [FakeBigQueryServices.java](https://github.com/apache/beam/blob/master/sdks/java/io/google-cloud-platform/src/test/java/org/apache/beam/sdk/io/gcp/bigquery/FakeBigQueryServices.java)
* [FakeJobService.java](https://github.com/apache/beam/blob/release-2.4.0/sdks/java/io/google-cloud-platform/src/test/java/org/apache/beam/sdk/io/gcp/bigquery/FakeJobService.java)
* etc

when we can create FakeBigQueryServices object using FakeDatasetService and FakeJobService. The FakeBigQueryServices should be used with BigQueryIO using withTestServices in order to test in a fake/mock way.

The BigQueryIO has withTestServices annotated as  VisibleForTesting, that means if anybody would like to use FakeBigQueryServices they have to create test cases in test cases under  org.apache.beam.sdk.io.gcp.bigquery package only.

It makes sense to provide these FakeBigQueryServices and testing BigQueryIO, in a way that it can be used by application builder to test them from their application code with any package implementation. 

Imported from Jira [BEAM-6642](https://issues.apache.org/jira/browse/BEAM-6642). Original Jira may contain additional context.
Reported by: asethia.

Read sdks/java/io/google-cloud-platform/src/test/java/org/apache/beam/sdk/io/gcp/bigquery/FakeBigQueryServices.java and FakeJobService.java, then inspect BigQueryIO.withTestServices and its visibility. Determine how application code outside the BigQuery package can use the fake services; done means external-package tests can test BigQueryIO with FakeBigQueryServices.

Written by the indexing model from the issue text.