[metricbeat][6.8][windows] TestData

## Flaky Test

* **Test Name:** TestData
* **Branch:** 6.8
* **Artifact Link:** https://beats-ci.elastic.co/blue/organizations/jenkins/Beats%2Fbeats%2FPR-21859/detail/PR-21859/10/pipeline#step-2671-log-1
* **Notes:** Additional details about the test. e.g. theory as to failure cause

### Stack Trace

```
[2020-10-22T14:41:52.581Z]

[2020-10-22T14:41:52.581Z] C:\Users\jenkins\workspace\Beats_beats_PR-21859\src\github.com\elastic\beats\metricbeat>mage fields build goTestUnit

[2020-10-22T14:41:59.404Z] Generated fields.yml for metricbeat to C:\Users\jenkins\workspace\Beats_beats_PR-21859\src\github.com\elastic\beats\metricbeat\fields.yml

[2020-10-22T14:41:59.404Z] >> build: Building metricbeat

[2020-10-22T14:42:46.934Z] >> go test: Unit Testing

[2020-10-22T14:44:08.391Z] FAILURES:

[2020-10-22T14:44:08.391Z] Package: github.com/elastic/beats/metricbeat/module/windows/perfmon

[2020-10-22T14:44:08.391Z] Test: TestData

[2020-10-22T14:44:08.391Z] modules.go:94: failed to create new MetricSet 1 error: initialization of reader failed: failed to add counter (query="\FileSystem Disk Activity(_Total)\FileSystem Bytes Read"): The specified object was not found on the computer.

[2020-10-22T14:44:08.391Z] ----

[2020-10-22T14:44:08.391Z] SUMMARY:

[2020-10-22T14:44:08.391Z] Fail: 1

[2020-10-22T14:44:08.391Z] Skip: 21

[2020-10-22T14:44:08.391Z] Pass: 319

[2020-10-22T14:44:08.391Z] Packages: 107

[2020-10-22T14:44:08.391Z] Duration: 1m10.5292968s

[2020-10-22T14:44:08.391Z] Coverage Report: C:\Users\jenkins\workspace\Beats_beats_PR-21859\src\github.com\elastic\beats\metricbeat\build\TEST-go-unit.html

[2020-10-22T14:44:08.391Z] JUnit Report: C:\Users\jenkins\workspace\Beats_beats_PR-21859\src\github.com\elastic\beats\metricbeat\build\TEST-go-unit.xml

[2020-10-22T14:44:08.391Z] Output File: C:\Users\jenkins\workspace\Beats_beats_PR-21859\src\github.com\elastic\beats\metricbeat\build\TEST-go-unit.out

[2020-10-22T14:44:08.391Z] >> go test: Unit Test Failed

[2020-10-22T14:44:08.391Z] Error: go test failed: 1 test failures

script returned exit code 1
```

Start in metricbeat/module/windows/perfmon at TestData and reproduce the failure with the Windows CI command shown in the stack trace: mage fields build goTestUnit. Compare the failing FileSystem Disk Activity counter with the test's expected Windows performance counters; done means TestData passes reliably without the missing-counter error.

Written by the indexing model from the issue text.