Enable process.include_cpu_ticks in system tests

By enabling the feature we will automatically be ensuring that the fields
in the event are also in the fields.yml.

(cherry picked from commit 93eedf3)

metricbeat/tests/system/test_system.py

@@ -377,7 +377,8 @@ def test_process(self):
             "metricsets": ["process"],
             "period": "5s",
             "extras": {
-                "process.env.whitelist": ["PATH"]
+                "process.env.whitelist": ["PATH"],
+                "process.include_cpu_ticks": True,
             }
         }])
         proc = self.start_beat()