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

[cypress-app-bot]

This issue has not had any activity in 180 days. Cypress evolves quickly and the reported behavior should be tested on the latest version of Cypress to verify the behavior is still occurring. It will be closed in 14 days if no updates are provided.