change timestamp to ISO 8601 format in test-case

Author: stefans-elastic

packages/system/data_stream/syslog/_dev/test/pipeline/test-iso8601-timestamp-multiline-syslog.log

@@ -1,3 +1,3 @@
-2020-12-25T23:59:59+02:00 a-mac-with-esc-key GoogleSoftwareUpdateAgent[21412]: 2016-12-13 11:35:28.421
+2022-04-21T14:30:00Z a-mac-with-esc-key GoogleSoftwareUpdateAgent[21412]: 2022-04-21T14:30:00Z
 		GoogleSoftwareUpdateAgent[21412/0x700007399000] [lvl=2] -[KSUpdateEngine updateAllExceptProduct:] KSUpdateEngine updating all
 		installed products, except:'com.google.Keystone'.

packages/system/data_stream/syslog/_dev/test/pipeline/test-iso8601-timestamp-multiline-syslog.log-expected.json

@@ -14,7 +14,7 @@
       "input": {
         "type": "log"
       },
-      "message": "2016-12-13 11:35:28.421\n\t\tGoogleSoftwareUpdateAgent[21412/0x700007399000] [lvl=2] -[KSUpdateEngine updateAllExceptProduct:] KSUpdateEngine updating all\n\t\tinstalled products, except:'com.google.Keystone'.",
+      "message": "2022-04-21T14:30:00Z\n\t\tGoogleSoftwareUpdateAgent[21412/0x700007399000] [lvl=2] -[KSUpdateEngine updateAllExceptProduct:] KSUpdateEngine updating all\n\t\tinstalled products, except:'com.google.Keystone'.",
       "process": {
         "name": "GoogleSoftwareUpdateAgent",
         "pid": 21412