This adds a new ignition_turbofan testing variant that'll
allow switching turbofan off in the other ignition variant.

The switch will happen in a follow up after reconfiguring
the bots.

BUG=v8:4280
LOG=N

Review URL: https://codereview.chromium.org/1922623003

Cr-Commit-Position: refs/heads/master@{#35790}