This commit makes asynchronous requests first class by providing a
MvcTestResult that represents the final, completed, state of a request
by default. Previously, the result was an intermediate step that may
require an ASYNC dispatch to be fully usable. Now it is de facto
immutable.
To make things a bit more explicit, an `.exchange(Duration)` method has
been added to provide a dedicated time to wait. The default applies a
default timeout that is consistent with what MVcResult#getAsyncResult
does.
Given that we apply the ASYNC dispatch automatically, the intermediate
response is no longer available by default. As a result, the asyncResult
is not available for assertions.
As always, it is possible to use plain MockMvc by using the `perform`
method that takes the regular RequestBuilder as an input. When this
method is invoked, no asynchronous handling is done.
Closes gh-33040
a7503e7200