fix System event log entry is not empty

Symptom:
When running full automation test cases, the test case
"Reboot And Check Ambient Temperature Records Are Valid" sometimes got failed.
The error log is "System event log entry is not empty".

Root cause:
The keyword "Get Thermal Records and Verify" will check whether there is any
error event log when checking ambient temperature records are valid.
However, all temperature records are valid in test result.
But, seems there are other unrelated system error event logs already exist.
Thus, cause this keyword return failed.

Solution:
Prevent this unexpected redfish event log when doing some test items.
We should clear redfish event log to make sure the error event is not cause
by current test item.

Tested:
Run robot test redfish/systems/test_thermal_ambient_temperatures.robot.

Signed-off-by: Tim Lee <timlee660101@gmail.com>
Change-Id: Id1d9d9bd2307f51be67e284f64373622b147cc5c
diff --git a/redfish/systems/test_thermal_ambient_temperatures.robot b/redfish/systems/test_thermal_ambient_temperatures.robot
index 4eb0226..8a79051 100755
--- a/redfish/systems/test_thermal_ambient_temperatures.robot
+++ b/redfish/systems/test_thermal_ambient_temperatures.robot
@@ -31,7 +31,10 @@
     [Tags]  Reboot_And_Check_Ambient_Temperature_Records_Are_Valid
 
     Redfish OBMC Reboot (run)
+
+    # Clear errors if there are any.
     Redfish.Login
+    Redfish Purge Event Log
 
     Get Thermal Records and Verify  Temperatures  ReadingCelsius