Migrate InitializationTests to addSourceLines (#1694)

## Summary

- replace the deprecated `addSourceFile` calls in `InitializationTests`
with inline `addSourceLines` text blocks
- factor the shared `Util.java` source into a helper, as requested in
the issue discussion
- remove the four testdata files that are no longer referenced and the
obsolete class-level deprecation suppression

Part of #1425.

## Testing

- `./gradlew :nullaway:spotlessJavaApply -Pspotless.ratchet.from=HEAD`
- `./gradlew :nullaway:test --tests
"com.uber.nullaway.InitializationTests"`

## AI usage disclosure

> I used OpenAI Codex to help identify the affected calls, migrate the
existing test sources into text blocks, factor the shared source in
response to maintainer feedback, run formatting and targeted tests, and
draft this PR description. I personally reviewed the full diff and
attest that I have read and understood all changes in this PR.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Tests**
* Consolidated initialization and read-before-initialization scenarios
directly into the test suite.
* Expanded coverage for field and static initialization, initializer
methods, framework annotations, suppression, nested classes, casts,
assertions, and early field reads.
* Removed redundant external test fixtures while preserving and
broadening diagnostic coverage.
* Improved consistency and maintainability of initialization-related
validation.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: abdeltaehass