Flaky Test [Build&Test / x-pack/heartbeat-windows-windows-2019 / TestRunCmd/has_echo'd_stdin_to_stdout – github.com/elastic/beats/v7/x-pack/heartbeat/monitors/browser/synthexec]

## Flaky Test

* **Test Name:** `TestRunCmd/has_echo'd_stdin_to_stdout – github.com/elastic/beats/v7/x-pack/heartbeat/monitors/browser/synthexec`
* **Link:** Link to file/line number in github.
* **Branch:** master
* **Artifact Link:** https://beats-ci.elastic.co//blue/rest/organizations/jenkins/pipelines/Beats/pipelines/beats/pipelines/PR-23873/runs/4/steps/1274/log/?start=0
* **Notes:** Platforms: only fails in Windows (all the versions)

### Stack Trace

```
Error
Failed
Stacktrace
=== RUN TestRunCmd/has_echo'd_stdin_to_stdout
--- FAIL: TestRunCmd/has_echo'd_stdin_to_stdout (0.00s)
panic: runtime error: index out of range [0] with length 0 [recovered]
panic: runtime error: index out of range [0] with length 0
goroutine 21 [running]:
testing.tRunner.func1.1(0x152b220, 0xc0004a6860)
C:/Users/jenkins/workspace/Beats_beats_PR-23873/.gvm/versions/go1.15.7.windows.amd64/src/testing/testing.go:1072 +0x310
testing.tRunner.func1(0xc00010a300)
C:/Users/jenkins/workspace/Beats_beats_PR-23873/.gvm/versions/go1.15.7.windows.amd64/src/testing/testing.go:1075 +0x43a
panic(0x152b220, 0xc0004a6860)
C:/Users/jenkins/workspace/Beats_beats_PR-23873/.gvm/versions/go1.15.7.windows.amd64/src/runtime/panic.go:969 +0x1c7
github.com/elastic/beats/v7/x-pack/heartbeat/monitors/browser/synthexec.TestRunCmd.func2(0xc00010a300)
C:/Users/jenkins/workspace/Beats_beats_PR-23873/src/github.com/elastic/beats/x-pack/heartbeat/monitors/browser/synthexec/synthexec_test.go:133 +0x136
testing.tRunner(0xc00010a300, 0xc00049c040)
C:/Users/jenkins/workspace/Beats_beats_PR-23873/.gvm/versions/go1.15.7.windows.amd64/src/testing/testing.go:1123 +0xef
created by testing.(*T).Run
C:/Users/jenkins/workspace/Beats_beats_PR-23873/.gvm/versions/go1.15.7.windows.amd64/src/testing/testing.go:1168 +0x2b3
```

```[tasklist]
### Tasks
- [ ] Uncomment the commented-out steps of the x-pack/heartbeat pipeline when it's done.
```