tests: fix flaky test in entity_store_compaction

[Bravo555]

Proposed changes

The test's check could run before compaction process is completed, failing it. I've observed in the test report, that after failure, entity store was 4 entities (and only 1 was the compaction_count device), so the compaction completed, just a little later.

To fix this, do the comparison using test in bash so it's subject to the retry policy. To confirm that it worked, I see the following in the report of some test runs after the fix:

cmd: ['/bin/sh', '-c', 'bash -c \'[ $(grep -c "compaction_count" /etc/tedge/.agent/entity_store.jsonl) -lt 10 ]\''], exit code: 1
...
cmd: ['/bin/sh', '-c', 'bash -c \'[ $(grep -c "compaction_count" /etc/tedge/.agent/entity_store.jsonl) -lt 10 ]\''], exit code: 0

The retry is applied once, the compaction completes, and the test succeeds.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (general improvements like code refactoring that doesn't explicitly fix a bug or add any new functionality)
• Documentation Update (if none of the other choices apply)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Paste Link to the issue

Checklist

• I have read the CONTRIBUTING doc
• I have signed the CLA (in all commits with git commit -s. You can activate automatic signing by running just prepare-dev once)
• I ran just format as mentioned in CODING_GUIDELINES
• I used just check as mentioned in CODING_GUIDELINES
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

[codecov]

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Robot Results

✅ Passed	❌ Failed	⏭️ Skipped	Total	Pass %	⏱️ Duration
953	0	7	953	100	2h50m55.760073s