Tests: Fix list tests so they logic isn't changed after incorporating…

[jodator]

Suggested merge commit message (convention)

Tests: Fix list tests so they logic isn't changed after incorporating selection post-fixer. Closes ckeditor/ckeditor5#3004.

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Additional information

• As @scofalik suggested I've reverted some changes in the test so their logic remains the same as originally written. Now the editor setup is done as a separate step then actionCallback that runs the actual test steps. The selection in tests is not set - the [] is parsed and passed to the actionCallback that runs the test.

[coveralls]

Coverage increased (+0.1%) to 100.0% when pulling 28af705 on t/107 into 7447b0d on master.