tests: Set environment variable USER if not set

[sjackman]

These two tests fail if the enivornment variable USER is not set:

./test/utils/user_spec.rb:6 # User should eq nil
./test/utils/user_spec.rb:22 # User#gui? when the current user is in a console session gui? should equal true

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The environment variable USER is not set in Docker by default.

[MikeMcQuaid]

Makes sense, thanks @sjackman!