[ML] CI failure in CKMostCorrelatedTest/testScale

Seen in https://elasticsearch-ci.elastic.co/job/elastic+machine-learning+pull-request+linux/1278/consoleText

```
2022-06-27 13:29:59,111091 UTC [4711] DEBUG /ml-cpp/lib/test/CTestObserver.cc@28 +----------------------------------+
2022-06-27 13:29:59,111102 UTC [4711] DEBUG /ml-cpp/lib/test/CTestObserver.cc@29 | CKMostCorrelatedTest/testScale |
2022-06-27 13:29:59,111112 UTC [4711] DEBUG /ml-cpp/lib/test/CTestObserver.cc@30 +----------------------------------+
2022-06-27 13:29:59,144512 UTC [4711] DEBUG /ml-cpp/lib/maths/common/unittest/CKMostCorrelatedTest.cc@751 elapsed time = 28ms
2022-06-27 13:29:59,240307 UTC [4711] DEBUG /ml-cpp/lib/maths/common/unittest/CKMostCorrelatedTest.cc@751 elapsed time = 88ms
2022-06-27 13:29:59,527676 UTC [4711] DEBUG /ml-cpp/lib/maths/common/unittest/CKMostCorrelatedTest.cc@751 elapsed time = 272ms
2022-06-27 13:30:00,362381 UTC [4711] DEBUG /ml-cpp/lib/maths/common/unittest/CKMostCorrelatedTest.cc@751 elapsed time = 805ms
2022-06-27 13:30:04,480209 UTC [4711] DEBUG /ml-cpp/lib/maths/common/unittest/CKMostCorrelatedTest.cc@751 elapsed time = 3980ms
2022-06-27 13:30:04,498173 UTC [4711] DEBUG /ml-cpp/lib/maths/common/unittest/CKMostCorrelatedTest.cc@754 elapsed times = [28, 88, 272, 805, 3980]
2022-06-27 13:30:04,498299 UTC [4711] DEBUG /ml-cpp/lib/maths/common/unittest/CKMostCorrelatedTest.cc@763 exponent = 1.82145
2022-06-27 13:30:04,498337 UTC [4711] DEBUG /ml-cpp/lib/maths/common/unittest/CKMostCorrelatedTest.cc@766 sdRatio = 0.266806
/ml-cpp/lib/maths/common/unittest/CKMostCorrelatedTest.cc(776): fatal error: in "CKMostCorrelatedTest/testScale": critical check exponent < 1.75 has failed [1.8214474079906533 >= 1.75]
2022-06-27 13:30:04,498726 UTC [4711] INFO /ml-cpp/lib/test/CTestObserver.cc@40 Unit test timing - CKMostCorrelatedTest/testScale took 5387ms
```

The test run was on Ubuntu 18.04 on linux-x86_64.

That test doesn't fail in every run, so something it does must produce non-deterministic results.

Iniziare da ml-cpp/lib/maths/common/unittest/CKMostCorrelatedTest.cc, in particolare da testScale intorno alle righe 751-776, e riprodurre il fallimento su Ubuntu 18.04 Linux x86_64 usando l'output di CI come riferimento. Analizzare perché le misurazioni del tempo trascorso e l'esponente calcolato variano tra un'esecuzione e l'altra; il lavoro è completato quando il test non fallisce più in modo non deterministico, mantenendo al contempo controlli di scalabilità significativi.

Scritto dal modello di indicizzazione a partire dal testo della issue.