[BEAM-9767]: Fix flaky streaming wordcount

[rohdesamuel]

The BackgroundCachingJob condition checker was failing due to a data race where the StreamingCache was created and read from before there was any data. This raised an uncaught StopIteration exception and failed the condition checker.

Also increases the timeout to 60s. On my local machine I reproed it by executing:

for i in {1..100}; do python interactive_runner_test.py InteractiveRunnerTest.test_streaming_wordcount 2>&1 | tee test_logs/$i & done

Which took a max of 60s.

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Post-Commit Tests Status (on master branch)

Lang	SDK	Apex	Dataflow	Flink	Gearpump	Samza	Spark
Go		---	---		---	---
Java
Python		---			---	---
XLang	---	---	---		---	---

Pre-Commit Tests Status (on master branch)

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

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

[rohdesamuel]

R: @pabloem

[pabloem]

retest this please

[rohdesamuel]

Run Python2_PVR_Flink PreCommit

[pabloem]

Run Python2_PVR_Flink PreCommit

[pabloem]

LGTM