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
1c87e4795d