Fix scenario selection

In response to issue #778

Previously, duplicate files within the `file_specs` where executed twice and
affected the execution order of scenarios. This change ensures only unique
files are returned from file specs.
Merge branch 'master' into fix-scenario-selection
Remove unnecessary acceptance test

This scenario is covered in the `FileSpecs` spec
Merge pull request #786 from jdks/fix-scenario-selection

Fix #778
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