Nightwatch

[vasco-sousa]

updated configuration of nightwatch configuration for continuous integration.

• Dropped the use of xvfb as that seemed to be the source of incompatibility
  • it runs properly if it is headless if it has a window size specified, otherwise it also fails as it did with xvfb
• Added the individual tests to the workflow for granularity
• Added command parameters to test script for convenience
  • using the keyword headless allows to run the tests in headless mode
  • passing a specific test file as the last argument for the convenience of not having to launch the servers by hand