Description

I recently discovered that MongoDB offers a `ephemeralForTest` engine.

This discovery followed issues my team experienced when upgrading the MongoDB instance used by our tests suite to v4.4, since we had to switch to `wiredTiger` as the engine.

Suddenly, our test suite was running considerably slower, causing delays in our development.

After days of tries to get this solved, from using `tmpfs` to disabling indexes and more, a member of our team finally found a reference to the `ephemeralForTest` engine.

I would have expected this engine to be mentioned in the Storage Engines page, but instead it is only mentioned in the v3.2 release notes.

Since this engine definitely solved our issue, I'd suggest it should be listed, or at least mentioned, in the Storage Engines page, to save other teams some precious time when they face this issue

Scope of changes

As mentioned in the ticket description, we should review the content on the ephemeral test engine mentioned in the v3.2 release notes here and see about moving them to the Storage Engine documentation.

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)