systemtest: ensure unique package policy names

[axw]

Motivation/summary

systemtest.TestTailSampling has been failing with errors like:

2021/11/12 05:31:37 Container is ready id: d7064c092de5 image: elastic-agent-systemtest:8.1.0-49c447b0-SNAPSHOT
    kibana.go:141: 
        	Error Trace:	kibana.go:141
        	            				fleet_test.go:104
        	            				sampling_test.go:69
        	Error:      	Received unexpected error:
        	            	There is already a package with the same name
        	Test:       	TestTailSampling

Initially we thought this was flakiness in our tests, then thought maybe it was a concurrency issue in CI. Actually, it's due to a functional change in Fleet: with elastic/kibana#115212 package policy names must not be globally unique, not just unique within their agent policy.

We now ensure package policy names we generate are globally unique by including the agent policy name.

Checklist

- [ ] Update CHANGELOG.asciidoc
- [ ] Documentation has been updated

How to test these changes

N/A

Related issues

None

[apmmachine]

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Build stats

• Start Time: 2021-11-14T02:17:45.833+0000

• Duration: 41 min 29 sec

• Commit: 8130863

Test stats 🧪

Test	Results
Failed	0
Passed	6036
Skipped	19
Total	6055

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