Reorganize secrets test to use only `isolation/abstract_unit`
Reorganize secrets test to use only `isolation/abstract_unit`

Currently, secrets test uses `abstract_unit` and `isolation/abstract_unit`.
This is a bit odd. Therefore, reorganize it so that use only
`isolation/abstract_unit`.

Context: https://github.com/rails/rails/pull/30520#issuecomment-327409586

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