Use headless option for cors functional test runner

Some selenium webdrivers (e.g. Chrome, Firefox) support a headless
option so we can expose that as an option for the test runner too.

Use this in an attempt to fix "Error: cannot open display: :99" errors
seen in the gate.

Related-Bug: #1918864
Change-Id: I2a549ce829eb0bc38406575582202e1d8dd1a0e2

test/cors/main.py

@@ -180,9 +180,19 @@ def run(args, url):
     browsers = list(ALL_BROWSERS) if 'all' in args.browsers else args.browsers
     ran_one = False
     for browser_name in browsers:
+        kwargs = {}
+        try:
+            options = getattr(
+                selenium.webdriver, browser_name.title() + 'Options')()
+            options.headless = True
+            kwargs['options'] = options
+        except AttributeError:
+            # not all browser types have Options class
+            pass
+
         driver = getattr(selenium.webdriver, browser_name.title())
         try:
-            browser = driver()
+            browser = driver(**kwargs)
         except Exception as e:
             if not ('needs to be in PATH' in str(e) or
                     'SafariDriver was not found' in str(e)):

tools/playbooks/cors/run.yaml

@@ -9,7 +9,7 @@
 
     - name: Run CORS tests
       shell: >
-        xvfb-run python
+        python
         {{ ansible_env.HOME }}/{{ zuul.project.src_dir }}/test/cors/main.py
         --output {{ ansible_env.HOME }}/cors-test-results.txt
         all