3.x Update FileTrait

[AlexSkrypnyk]

Given the following managed files:
Given the following managed files do not exist:
Given the unmanaged file at the URI :uri exists
Given the unmanaged file at the URI :uri exists with content:
Then an unmanaged file at the URI :uri should exist
Then an unmanaged file at the URI :uri should not exist
Then an unmanaged file at the URI :uri should contain :content
Then an unmanaged file at the URI :uri should not contain :content