dillon1000/react

Double Invoke Effects in __DEV__ (in old reconciler fork) (#20415)

We originally added a new DEV behavior of double-invoking effects during mount to our new reconciler fork in PRs #19523 and #19935 and later refined it to only affect modern roots in PR #20028. This PR adds that behavior to the old reconciler fork with a small twist– the behavior applies to StrictMode subtrees, regardless of the root type.

This commit also adds a few additional tests that weren't in the original commits.