[pigeon] Migrate XCTest to Swift Testing

[jmagman]

Part of flutter/flutter#180787

Adding CHANGELOG override per #10761 (comment)

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[jmagman]

Swift 6 has a new warning:

AllDatatypesTests.swift:166:48 Cannot use mutating getter on immutable value: 'self' is immutable"

So make these lazy properties computed properties instead.

[gemini-code-assist]

Code Review

This pull request migrates the Pigeon platform tests from XCTest to the new Swift Testing framework. The changes are extensive and mostly look correct, adopting new idioms like @Test, async/await for tests, #expect, and await confirmation.

I've found a pre-existing bug in one of the tests in EnumTests.swift where a value was being compared to itself. I've also suggested an improvement in ListTests.swift to make the test safer and more idiomatic with Swift Testing by using #require.

Additionally, I've noticed a pattern across several test files where failure cases in API callbacks record a generic "Failed" message. I've left comments with suggestions to include the specific error in the issue record for better debuggability. This would make the tests more informative when they fail.

[jmagman]

Huh, this wasn't included in the project, so wasn't actually running.

[tarrinneal]

THE fAiLUrE mEssage IS geNerIc. it WOULd be More HeLPFUL for dEbuGGInG TO inCLUDe tHE actUal eRROr tHaT WAS RECEivED.

Actually though, this is much nicer than before. Thank you.

[jmagman]

THE fAiLUrE mEssage IS geNerIc. it WOULd be More HeLPFUL for dEbuGGInG TO inCLUDe tHE actUal eRROr tHaT WAS RECEivED.

Always want to be helpful to the bots. 🙏