[BEAM-7754] 2.7.1 backport: fix DataflowRunnerTest expected error msg

[kennknowles]

This cherry-picks #8013 to the 2.7.1 release branch, to improve testing signal

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Post-Commit Tests Status (on master branch)

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

Pre-Commit Tests Status (on master branch)

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

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[kennknowles]

@ibzib @ryan-williams thanks for having this fix easy to find in the git history

[kennknowles]

Since this is obscuring test signal, policy is to merge first get LGTM later - I will do so after confirming it improves the test in question.

[kennknowles]

The examples failure is fixed by #9076.

[kennknowles]

Looks like this fixed the expected test.