Use embedded http server to test loading of remote images

[troosan]

Description

Tests sometimes fail because we are loading images from a remote server.
The PR will fire up the embedded php http server and load the images from there

Checklist:

• I have run composer run-script check --timeout=0 and no errors were reported
• The new code is covered by unit tests (check build/coverage for coverage report)

[coveralls]

Coverage decreased (-0.04%) to 94.533% when pulling c408ac5 on troosan:avoid_remote_image_loading_during_tests into 7b7d4e4 on PHPOffice:develop.