Some tests for TransformStream used the delay() function with non-zero
delays. Mostly this was done to make things happen asynchronously in a
particular order, and has been replaced with equivalent code using
promise chains.

The test "TransformStream flush is called after all queued writes
finish, once the writable is closed" was passing accidentally as it was
using test() where it should have been using promise_test(). In addition
to removing non-zero delays from the test, also fix it to use
promise_test() and remove the backpressure that was causing transform()
to never be called.

See https://github.com/whatwg/streams/issues/548 for background on this
clean-up.