Currently the `HttpClient` always wraps errors from XHR requests, but
the underlying errors are always of type `ProgressEvent`, or don't have
a native error if the status code is just indicating failure (e.g. 404).

This behavior does not match in the `TestRequest` class provided by
`@angular/common/http/testing` where errors are considered being
of type `ErrorEvent`. This is incorrect because `ErrorEvent`s provide
information for errors in scripts or files which are evaluated. Since
the `HttpClient` never evaluates scripts/files, and also since XHR requests
clearly are documented to emit `ProgressEvent`'s, we should change the
`TestSupport` to retrieve such `ProgressEvent`'s instead of incompatible
objects of type `ErrorEvent`.

In favor of having a deprecation period, we keep supporting `ErrorEvent`
in the `TestRequest.error` signature. Eventually, we can remove this
signature in the future.

Resources:
  * https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/error_event
  * https://developer.mozilla.org/en-US/docs/Web/API/ErrorEvent
  * https://xhr.spec.whatwg.org/#event-xhr-errpr

Related to: angular#34748.

DEPRECATED: `TestRequest` from `@angular/common/http/testing` no longer
accepts `ErrorEvent` when simulating XHR errors. Instead instances of
`ProgressEvent` should be passed, matching with the native browser behavior.