Attempt to fix flaky shutdown test

[artem-shelkovnikov]

Maybe closes #2517

Not sure about the actual reason for the test to inconsistently fail, but the steps that I add generally don't make it worse:

1. Change makes use of pre-defined test config file instead of using default one. There should not even be a default file on the host, but test depending on ephemereal config file is doomed to be flaky
2. Mock Elasticsearch calls instead of making service attempt call it. This might actually be the problem - service keeps trying to connect and ignore shutdown procedure, let's see.

Due to 2. this test might actually show potential problem with shutdown for service that tries to connect to something non-existent while terminating at a very specific point in time, but I was unable to reproduce it locally.

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