tests: Disable tests that require being non-root

[sjackman]

Set the environment variable USER=root which is required by
brew tests --only=utils/user

Fix these tests:

rspec ./test/diagnostic_checks_spec.rb:26 # Homebrew::Diagnostic::Checks #check_access_directories
rspec ./test/pathname_spec.rb:120 # Pathname#ensure_writable makes a file writable and restores permissions
rspec ./test/utils/user_spec.rb:22 # User#gui? when the current user is in a console session gui? should equal true
rspec ./test/utils/user_spec.rb:6 # User should eq nil

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?

[sjackman]

I'll merge this PR after CI is 💚. Hopefully that fixes the failed build on Docker Hub.
https://cloud.docker.com/u/linuxbrew/repository/docker/linuxbrew/brew/builds

[iMichka]

Thanks @sjackman

[sjackman]

Thanks for merging, Michka!

[sjackman]

Docker Hub build failed with…

==> brew update-test --to-tag
==> FAILED
Start commit: 1facac803ee3e4bfe78adcbff5dcd476abc1f378
End commit: d27320087aa8f8a6653db93ff925e7b9c05bea8d
==> Setup test environment...
Cloning into '.'...
done.
Cloning into bare repository 'remote.git'...
done.
Reset branch 'master'
Your branch is up to date with 'origin/master'.
Everything up-to-date
HEAD is now at 1920eecb7 Merge pull request #5651 from retokromer/patch-1
==> Running brew update...
Error: Running Homebrew as root is extremely dangerous and no longer supported.
As Homebrew does not drop privileges on installation you would be giving all
build scripts full access to your system.
Error: Failure while executing; `brew update --verbose` exited with 1.

https://cloud.docker.com/u/linuxbrew/repository/registry-1.docker.io/linuxbrew/brew/builds/af752a01-a332-45f4-acfb-5680d175b592