Move AbsoluteAssetPathError under tests for runtime errors

The test for `AbsoluteAssetPathError` didn't set the `Sprockets::Rails::Helper.precompile` array, which leads to errors since `foo.js` is not included when calling `asset_path`.

This change moves the test for `AbsoluteAssetPathError` from `ManifestHelperTest` to `RuntimeErrorsHelperTest` and explicitly sets the `precompile` array.