Post-processing of mocked beans causes a number of problems:

 - The mock may be proxied for asynchronous processing which can cause
   problems when configuring expectations on a mock (gh-6573)
 - The mock may be proxied so that its return values can be cached or
   so that its methods can be transactional. This causes problems with
   verification of the expected calls to a mock (gh-6573, gh-5837)
 - If the mock is created from a class that uses field injection, the
   container will attempt to inject values into its fields. This causes
   problems if the mock is being created to avoid the use of one of
   those dependencies (gh-6663)
 - Proxying a mocked bean can lead to a JDK proxy being created
   (if proxyTargetClass=false) as the mock implements a Mockito
   interface. This can then cause injection failures as the types don’t
   match (gh-6405, gh-6665)

All of these problems can be avoided if a mocked bean is not
post-processed. Avoiding post-processing prevents proxies from being
created and autowiring from being performed. This commit avoids
post-processing by registering mocked beans as singletons as well as
via a bean definition. The latter is still used by the context for type
matching purposes.

Closes gh-6573, gh-6663, gh-6664