[8.16](backport #41580) [chore][metricbeat/test_diskio]: fix CI failure

[mergify]

For diskio metricset,

beats/metricbeat/module/system/diskio/diskio.go

Lines 118 to 120 in bfaa70f

	if counters.SerialNumber != "" {
	event["serial_number"] = counters.SerialNumber
	}

serial_number field is set, conditionally.

This error has likely originated by bumping PR on https://github.com/elastic/elastic-agent-system-metrics.

I think, for such conditional fields, our best case is to remove them from event or else they trigger a failure.

We cannot add such fields at

beats/metricbeat/module/system/test_system.py

Lines 70 to 76 in bfaa70f

	SYSTEM_DISKIO = {
	metricbeat.P_DEF: ["name", "read.count", "write.count", "read.bytes",
	"write.bytes", "read.time", "write.time"]
	}
	SYSTEM_DISKIO[metricbeat.P_LINUX] = SYSTEM_DISKIO[metricbeat.P_DEF] + \
	["io.time", "io.ops"]

as that would also result in a failure, because the serial_number can be absent for some devices.

This is needed to unblock #41524

---

This is an automatic backport of pull request #41580 done by Mergify.

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)