Lazy load IllustrationManager

[trasher]

IllustrationManager requires files to be present at it instanciation; which assumes node deps have been installed.
For the CI of a plugin, I install GLPI and composer deps only, to run check headers command:

Run cd glpi && php bin/console tools:licence_headers_check --plugin glpiinventory

RuntimeException {#7520
  #message: "Failed to read file: /home/runner/work/glpi-inventory-plugin/glpi-inventory-plugin/glpi/public/lib/glpi-project/illustrations/icons.json"
  #code: 0
  #file: "./src/Glpi/UI/IllustrationManager.php"
  #line: 419
  [...]

Of course I could just install npm deps plugin side, but that's not required, and that would take more time jusst for a lint job.
It seems also better this error not to happen in the constructor.

PR also fix a test that failed several times during my tests.