Saas 4 report fixes sle

Bunch of fixes for saas-4 report module including
* addresses blocks more to the left on all saas-4 external reports
* do not raise wkhtmltopdf on contentnotfounderror
* removed the very ugly worker creation when printing a report when Odoo is running with only one worker (now fallback on html mode)
* do not generate pdf (only html) when running the tests without a test report dir
* fetch the wkhtmltopdf version in a cleaner way (and working on macos release)