Ensure Selenium Test Suite Compatibility with Ubuntu Server Environment

[shinair]

Describe the problem

The current OpenEMR Selenium Test suit needs changes so it can run headless on the Ubuntu server.

Write a concise description of the problem:
Since the Ubuntu Server is headless, the tests need to run without a GUI. This needs to be specific in the tests in addition to passing a command-line argument while running the Python test file.

Describe the solution you'd like

Make changes to the tests such that they run headless and update the readme.

Write concise description of what feature you want:

1. The tests should run headless
2. The readme should reflect the changes accordingly.

[shinair]

PR that addresses this issue:
Fixed vitals validation bug, modified tests and readme instructions to run on web18 #11
#11