rename server-action.test.ts to waku.test.ts for better clarity

The test file covers broader RSC transform functionality ported from Waku's
vite-plugin-rsc-transform-internals.test.ts, not just server actions.
It tests server actions, client references, and various RSC patterns.

The name 'waku.test.ts' better reflects that this is a comprehensive port
of Waku's RSC transform test cases for compatibility verification.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: hi-ogawa