Do not allow numTestsKeptInMemory to be set when headless

[ryanthemanuel]

What would you like?

Today, we set numTestsKeptInMemory to 0 regardless when in run mode. However, there's still a hole where a user can use setupNodeEvents to set something other than 0 and the value sticks.

Why is this needed?

This prevents us from capturing snapshots properly when recording to protocol, so we should only allow non 0 values in situations where it's likely to be helpful (open mode or headed).

Other

No response