Future vite versions won't accept JSX inside js files anymore (as they should!)

chore: rename `.test.js` -> `.test.jsx` when JSX is used

chore: migrate remaining .js -> .jsx test files

chore: speed up tests by using playwright instead of webdriverio

…ys (#5065) (#5067)

When a VNode is moved via INSERT_VNODE during diffChildren, its old _dom
pointer becomes a stale positional reference. Nested diffs that call
getDomSibling would traverse the old VNode tree and find this stale _dom,
causing subsequent DOM insertions at the wrong position.

Clear the old VNode's _dom after insert so getDomSibling skips moved
VNodes and finds the correct insertion point.