Move AbsoluteAssetPathError under tests for runtime errors

The test for `AbsoluteAssetPathError` doesn't set the `Sprockets::Rails::Helper.precompile` array, which leads to errors since `foo.js` is not included in the array when calling `asset_path` in https://github.com/rails/sprockets-rails/blob/master/test/test_helper.rb#L380.

This change moves the test for `AbsoluteAssetPathError` from `ManifestHelperTest` to `RuntimeErrorsHelperTest` and explicitly sets the `precompile` array.

This should fix the broken build.