[system tests] Use datastream dataset if it exists

[klacabane]

Summary

The system tests didn't account for the overwriting dataset defined in a data stream's manifest.

For example the logstash.node data stream that overwrites the dataset fails with the following:

Error: error running package system tests: could not complete test run: could not add data stream config to policy: could not add package to policy; API status code = 500; response body = {"statusCode":500,"error":"Internal Server Error","message":"Stream template not found, unable to find dataset logstash.node"}

[elasticmachine]

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Build stats

• Start Time: 2022-09-20T21:00:05.303+0000

• Duration: 33 min 53 sec

Test stats 🧪

Test	Results
Failed	0
Passed	832
Skipped	0
Total	832

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[elasticmachine]

🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (34/34)	💚
Files	66.929% (85/127)	👍
Classes	61.667% (111/180)	👍
Methods	49.044% (359/732)	👍 0.21
Lines	32.231% (3225/10006)	👍
Conditionals	100.0% (0/0)	💚

[jsoriano]

Change looks good, could you please add a package using dataset to test/packages? Thanks!

[jsoriano]

Thanks!