[BEAM-13430] Ensure that testRuntimeMigration depends on "default" configuration allowing us to get the main "jar" as a dependency.

[lukecwik]

This better mirrors what the "shadow" and "shadowTest" configurations do and what "testRuntime" used to do with gradle 6.x and earlier.

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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.

[lukecwik]

R: @ibzib

[ibzib]

LGTM

[lukecwik]

Run Python PreCommit

[lukecwik]

Run Python PreCommit