test: ignore streaming metadata in toBeEmpty assertions (#78456)
Some tests in `rsc-basic` asserts that certain nodes should be empty,
but streaming metadata can get in there and break that expectation.
- i switched the test to use a full browser. this is better than
checking the raw html, because then react gets rid of any `<template>`
tags inside the div (which are used for streaming SSR things)
- we're also now only asserting that the inner text of the node should
be empty, which avoids failures because of a `<div hidden>` inserted by
streaming metadata. this is a slightly weaker assertion than "this node
has no children at all" but it should be enough for our purposes here.
also reverts #78449 and #78424, they were meant for debugging this issue