[9.2] Make the test more flexible to entries that might have been rolled over (elastic#240072) (elastic#240240)

# Backport

This will backport the following commits from `main` to `9.2`:
- [Make the test more flexible to entries that might have been rolled
over (elastic#240072)](elastic#240072)

<!--- Backport version: 9.6.6 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sorenlouv/backport)

<!--BACKPORT [{"author":{"name":"Gerard
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(elastic#240072)\n\n## Summary\n\nCloses
https://github.com/elastic/kibana/issues/232893\n\nDespite the complex
time synchronization mechanism, the rollover can\nhappen right in the
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https://github.com/elastic/kibana/issues/232893\n\nDespite the complex
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the test more flexible to entries that might have been rolled over
(elastic#240072)\n\n## Summary\n\nCloses
https://github.com/elastic/kibana/issues/232893\n\nDespite the complex
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BACKPORT-->

Co-authored-by: Gerard Soldevila <[email protected]>

Author: kibanamachine

src/core/server/integration_tests/logging/rolling_file_appender.test.ts

@@ -303,13 +303,27 @@ describe('RollingFileAppender', () => {
       logger.info(message(3));
       logger.info(message(4));
 
+      await waitForNextRollingTime();
+
+      logger.info(message(5));
+      logger.info(message(6));
+
       await flush();
 
       const files = await readdir(testDir);
 
-      expect(files.sort()).toEqual(['kibana-1.log', 'kibana.log']);
-      expect(await getFileContent('kibana.log')).toEqual(expectedFileContent([3, 4]));
-      expect(await getFileContent('kibana-1.log')).toEqual(expectedFileContent([1, 2]));
+      const sortedLogFiles = files.sort();
+      expect(sortedLogFiles).toEqual(['kibana-1.log', 'kibana-2.log', 'kibana.log']);
+
+      // the "rolling time sync" mechanism above is not perfect, so the test is going to guarantee:
+      // - that something has been rolled over
+      // - that no log entries have been lost
+      const oldestEntries = await getFileContent('kibana-2.log');
+      const newerEntries = await getFileContent('kibana-1.log');
+      const newestEntries = await getFileContent('kibana.log');
+      const allEntries = oldestEntries + newerEntries + newestEntries;
+      expect(oldestEntries.length + newerEntries.length).toBeGreaterThan(40);
+      expect(allEntries).toEqual(expectedFileContent([1, 2, 3, 4, 5, 6]));
     });
   });
 });