Introduce enforceOverride flag in @⁠TestBean and @⁠MockitoBean

Prior to this commit, @⁠MockitoBean could be used to either create or
replace a bean definition, but @⁠TestBean could only be used to replace
a bean definition.

However, Bean Override implementations should require the presence of
an existing bean definition by default (i.e. literally "override" by
default), while giving the user the option to have a new bean
definition created if desired.

To address that, this commit introduces a new `enforceOverride`
attribute in @⁠TestBean and @⁠MockitoBean that defaults to true but
allows the user to decide if it's OK to create a bean for a nonexistent
bean definition.

Closes gh-33613

framework-docs/modules/ROOT/pages/testing/annotations/integration-spring/annotation-mockitobean.adoc

@@ -11,6 +11,10 @@ to override. If multiple candidates match, `@Qualifier` can be provided to narro
 candidate to override. Alternatively, a candidate whose bean definition name matches the
 name of the field will match.
 
+When using `@MockitoBean`, if you would like for a new bean definition to be created when
+a corresponding bean definition does not exist, set the `enforceOverride` attribute to
+`false` – for example, `@MockitoBean(enforceOverride = false)`.
+
 To use a by-name override rather than a by-type override, specify the `name` attribute
 of the annotation.
 

framework-docs/modules/ROOT/pages/testing/annotations/integration-spring/annotation-testbean.adoc

@@ -15,6 +15,10 @@ to override. If multiple candidates match, `@Qualifier` can be provided to narro
 candidate to override. Alternatively, a candidate whose bean definition name matches the
 name of the field will match.
 
+If you would like for a new bean definition to be created when a corresponding bean
+definition does not exist, set the `enforceOverride` attribute to `false` – for example,
+`@TestBean(enforceOverride = false)`.
+
 To use a by-name override rather than a by-type override, specify the `name` attribute
 of the annotation.