fix(test): Ensure `addEventListener` exists before invoking.
fix(test): Ensure `addEventListener` exists before invoking.

If `addEventListener` is not yet available on the window where the
tests are being run, try again after a short timeout.

fixes #3408

shane-tomlinson/fxa-content-server,mozilla/fxa-content-server,vladikoff/fxa-content-server,shane-tomlinson/fxa-content-server,swatilk/fxa-content-server,vladikoff/fxa-content-server,swatilk/fxa-content-server,mozilla/fxa-content-server,vladikoff/fxa-content-server,mozilla/fxa-content-server,swatilk/fxa-content-server,shane-tomlinson/fxa-content-server