Reject non-singletons in Test Bean Override support

Prior to this commit, a non-singleton FactoryBean was silently replaced
by a singleton bean. In addition, bean definitions for prototype-scoped
and custom-scoped beans were replaced by singleton bean definitions
that were incapable of creating the desired bean instance. For example,
if the bean type of the original bean definition was a concrete class,
an attempt was made to invoke the default constructor which either
succeeded with undesirable results or failed with an exception if the
bean type did not have a default constructor. If the bean type of the
original bean definition was an interface or a FactoryBean that claimed
to create a bean of a certain interface type, an attempt was made to
instantiate the interface which always failed with a
BeanCreationException.

To address the aforementioned issues, this commit reworks the logic in
BeanOverrideBeanFactoryPostProcessor so that an exception is thrown
whenever an attempt is made to override a non-singleton bean.

Closes gh-33602

framework-docs/modules/ROOT/pages/testing/testcontext-framework/bean-overriding.adoc

@@ -69,3 +69,6 @@ Alternatively, the user can directly provide the bean name in the custom annotat
 Some `BeanOverrideProcessor` implementations could also internally compute a bean name
 based on a convention or another advanced method.
 ====
+
+NOTE: Only _singleton_ beans can be overridden. Any attempt to override a non-singleton
+bean will result in an exception.