refactor-864370: Use Task.spawn in tests
refactor-864370: Use Task.spawn in tests

The header that we add to our tests as we port them to run in Mochitest
now uses Task.spawn rather than helpers.addTabWithToolbar

Signed-off-by: Joe Walker <7a17872fbb32c7d760c9a16ee3076daead11efcf@mozilla.com>

joewalker/gcli,mozilla/gcli,mozilla/gcli,joewalker/gcli,mozilla/gcli