[BEAM-11797] Fixed the flaky test

[nika-qubit]

1. Fixed the flaky test by explicitly assert_called_with rendered
   HTML/Javascript strings.
2. The flakiness could be introduced from other display calls emitted
   from the IPythonLogHandler though couldn't reproduce it locally.
   To rule out the flakiness and since assertion at a lower level is
   added, assertion around display is discarded.

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Post-Commit Tests Status (on master branch)

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

Pre-Commit Tests Status (on master branch)

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

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GitHub Actions Tests Status (on master branch)

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[nika-qubit]

R: @tvalentyn

PTAL, thx!

[codecov]

Codecov Report

Merging #14220 (e46bd86) into master (adf3dd4) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #14220   +/-   ##
=======================================
  Coverage   83.15%   83.15%           
=======================================
  Files         469      469           
  Lines       58972    58972           
=======================================
+ Hits        49036    49039    +3     
+ Misses       9936     9933    -3     

Impacted Files	Coverage Δ
...rcs/sdks/python/apache_beam/runners/direct/util.py
...ers/dataflow/dataflow_exercise_metrics_pipeline.py
...sdks/python/apache_beam/examples/flink/__init__.py
...apache_beam/examples/cookbook/custom_ptransform.py
...s/python/apache_beam/testing/synthetic_pipeline.py
...beam/testing/benchmarks/nexmark/queries/query11.py
...on/apache_beam/runners/portability/flink_runner.py
...x/py38/build/srcs/sdks/python/apache_beam/error.py
.../srcs/sdks/python/apache_beam/utils/sharded_key.py
...d/srcs/sdks/python/apache_beam/metrics/__init__.py
... and 928 more

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[tvalentyn]

Thanks!