fix(ext/node): run top-level node:test body before post-decl nextTick
node:test scheduled its top-level run via setTimeout(0), a macrotask that
runs after any process.nextTick / setImmediate callbacks queued by
synchronous code following the test() declaration. Node runs a top-level
test body before those. Schedule the run via process.nextTick instead: it
still lets all synchronous top-level test() calls finish registering, but
starts before post-declaration nextTick/setImmediate — matching Node.
Closes #35608