Rename tests to have the `.test.ts` extension

Would also require changes on the plugin-side.

Find all test files in the project, likely in a test/ directory, and rename them from .ts to .test.ts. Also locate the plugin configuration (maybe in hardhat.config.ts) to update any test file pattern references. Verify the changes by running the test suite to ensure they still execute correctly.

索引モデルが issue の本文から書いたものです。