[BEAM-12765] Fix broken BQ Integration Test

[satybald]

Fixes a broken test #15237

Lesson learned: specify TestDirectRunner instead of DirectRunner for integration tests

Command Tested:

pytest --log-cli-level=INFO --test-pipeline-options='--region=us-central1 --experiment=use_beam_bq_sink --runner=TestDirectRunner --project=....' -v apache_beam/io/gcp/bigquery_write_it_test.py

....
=================================================================================== 5 passed, 25 warnings in 127.29 seconds ===================================================================================

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ValidatesRunner compliance status (on master branch)

Lang	ULR	Dataflow	Flink	Samza	Spark	Twister2
Go	---					---
Java
Python	---					---
XLang						---

Examples testing status on various runners

Lang	ULR	Dataflow	Flink	Samza	Spark	Twister2
Go	---	---	---	---	---	---	---
Java	---		---	---	---	---	---
Python	---	---	---	---	---	---	---
XLang	---	---	---	---	---	---	---

Post-Commit SDK/Transform Integration Tests Status (on master branch)

Go	Java	Python

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website	Whitespace	Typescript
Non-portable
Portable	---			---	---	---

See .test-infra/jenkins/README for trigger phrase, status and link of all Jenkins jobs.

GitHub Actions Tests Status (on master branch)

See CI.md for more information about GitHub Actions CI.

[satybald]

Run Python PreCommit

[satybald]

Run Python 3.8 PostCommit

[codecov]

Codecov Report

Merging #15352 (6055eb6) into master (a0fbe00) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #15352      +/-   ##
==========================================
+ Coverage   83.80%   83.81%   +0.01%     
==========================================
  Files         441      440       -1     
  Lines       59801    59795       -6     
==========================================
+ Hits        50115    50117       +2     
+ Misses       9686     9678       -8     

Impacted Files	Coverage Δ
sdks/python/apache_beam/testing/test_pipeline.py	98.24% <ø> (ø)
setup.py	0.00% <0.00%> (ø)
test_config.py
...ks/python/apache_beam/runners/worker/sdk_worker.py	89.00% <0.00%> (+0.15%)	⬆️
sdks/python/apache_beam/runners/direct/executor.py	96.79% <0.00%> (+0.53%)	⬆️
sdks/python/apache_beam/io/source_test_utils.py	89.86% <0.00%> (+1.38%)	⬆️

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[pabloem]

LGTM. Thanks so much @satybald - I appreciate your fixing this : )

Other failed tests are broken on the master branch.