Precompile the image we're referencing, too.

You can't compile a file that references a non-compiled asset's path.

.. unless you turn off asset runtime errors.

Outside of a test case like this, `config.assets.precompile` would
normally retain its default entry, which precompiles all images (and
other non-JS/CSS files) that are in `app/assets`.
Merge pull request #14852 from matthewd/fix_build_for_assets

Fix railties asset tests: precompile the image we're referencing