* refactor: introduce Fixtures util class

* refactor: align yarn.spec

* refactor: move fs mocks to __mocks__ folder

* refactor: improve fs-extra mock

* refactor: move callsite to dev deps

* refactor: use fixtures class

* refactor: use fixtures class

* refactor: reduce external variables in tests

* refactor: improve Fixtures

* refactor: add type for realFs

* refactor: remove obsolete snapshot

* fix: by comments

* refactor: provide ability to spy on mocked fs methods

* refactor: fix build

* refactor: fix by comments

* refactor: add docs

* refactor: fix unit test

* refactor: return called times checks

* refactor: fix by comments

* refactor: adjust unit test

* refactor: adjust unit test

* refactor: fix unit test

* refactor: fix by comments

* refactor: fix by comments

* refactor: update jsdoc

* refactor: fix by comments

Co-authored-by: Rhys Arkins <rhys@arkins.net>
Co-authored-by: Jamie Magee <jamie.magee@gmail.com>
Co-authored-by: Michael Kriese <michael.kriese@visualon.de>