The fix for gh-20916 updated DefinitionsParser so that the
ResolvableType for each MockBean or SpyBean field included the
implementation class from which the field was found. Where the field
was declared with a variable generic signature that was made constant
by its implementation class, this allowed the correct concrete type to
be determined. It also had the unintended side-effect of preventing two
test classes with identical `@MockBean` and `@SpyBean` configuration
from sharing a context as the resolvable types for their mock and spy
bean fields would now be different.

This commit updates DefinitionsParser to only include the
implementation class in the ResolvableType if the field's generic type
is variable. For cases where it is not variable, this restores the
behaviour prior to the fix for gh-20916.

Fixes gh-22566