[BEAM-8244] Don't run external transform tests with pre_optimize=all.

[ibzib]

Fixes failures introduced in #12286

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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
Non-portable
Portable	---		---	---

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