[CI] CStringStoreTest.testStringStore failed

Build: https://elasticsearch-ci.elastic.co/job/elastic+machine-learning+pull-request/380/

Branches: 7.12

Failure:

```
[Exception] - critical check strings.size() == CStringStore::names().m_Strings.size() has failed [16 != 15]
 == [File] - CStringStoreTest.cc
 == [Line] -167
```

