Investigate model test failures in CI

[davidkyle]

The tests added in #623 can cause occasional build failures. Sometimes the console log just ends without any errors and sometimes there is a message along the lines of pytest failed with exit code -9.

The problem appears to be due to memory consumption as the tests pass when building on a larger machine. For example the tests passed when 52GB machine was used in https://github.com/elastic/eland/pull/625/commits/a98887714da98aa390cc3998e3d097653b76ea27

That change was not merged as build should not require so much memory.