[FIX] web(_editor): enable mobile tests

This rev. introduces a new test suite meant to test the webclient
components on mobile devices. The key 'config.device.isMobile' is
forced to true in this test suite, so that mobile specific JS files
are properly executed, which isn't the case in the classic JS test
suite (setting isMobile to true in the test definition is too late,
as the JS files are already processed).

For now, this new test suite contains a single test, which was
skipped until this rev. as it couldn't be executed in the classical
JS test suite.

Both suites are executed at each build of the runbot, and they
can be manually executed from the webclient as well (via the debug
manager).