Replace `eraseTime`, `eraseLapse` and `sliceLines` with custom messages

As of #260, we support overriding timestamp and messages with the `.gulp.js` file. We should remove the `eraseTime`, `eraseLapse`, and `sliceLines` test utilities and instead override timestamps and messages per test.

Start by locating the test utilities named `eraseTime`, `eraseLapse`, and `sliceLines`, then read the tests that use them and the `.gulp.js` override support from #260. Replace those utility uses with per-test timestamp and message overrides, and confirm the affected tests still pass without the utilities.

索引モデルが issue の本文から書いたものです。