chore: add note

TESTING.md

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+## Testing Notes
+
+> Quick notes on the findings when building the test, not a comprehensive guide yet.
+
+### On using Vitest
+
+- Although this project uses bun for the package manager, Vitest is used for testing solely because of its [typechecking capabilities](https://vitest.dev/guide/testing-types).
+
+- When running Vitest using Bun (`bun vitest run`), there are still `node` calls occurring in the background. Therefore, ensure that you specify `bunx prisma-generator-drizzle` when building the Prisma schema.
+
+- We're considering a full migration to `pnpm` + `vitest` in the future for the sake of consistency.