FOSS spec should not prepend EE extention

Problem

Currently, we're testing EE prepended class in FOSS spec (spec/*_spec.rb). This is problematic that, FOSS pipelines might not pass because the actual subject is different.

For instance, we have the following classes

# Base class (FOSS)
class TodosFinder
  ...
end

TodosFinder.prepend_if_ee('EE::TodosFinder')

# EE extension (proprietary)
class EE::TodosFinder
  ...
end

and when we run a spec for these classes, both subjects will be EE extended class

# Base class (FOSS)
describe TodosFinder do
  # This is not good because the subject is EE extended class.
end

# EE extension (proprietary)
describe TodosFinder do
  # This is OK
end