Test files in guest iframe are included via absolute path

[f1ames]

Trobulesome line https://github.com/cksource/karma-ckeditor4-bender-iframes/blob/79caea483d529bd8dcae79120a7f8e4125978874/src/host.js#L59.

While test files in main window are included directly by Karma using relative path (base/tests/...), in the iframe they are included using absolute path (which may break a Karm run) generated by preprocessor.

Test files in iframe should be also included via relative path same as in parent window.