Run dbaas_core tests with limited concurrency

[b1ron]

Closes #424.

I did not investigate all failures as it was taking too much time. I will create new issues as I go, e.g. "investigate X failures".

[b1ron]

@AlekSi just FYI there's around 100 core tests that rely on feature flag server parameters.

> const params = db.adminCommand( { getParameter: "*" } );
> params.featureFlagCollModIndexUnique
{ "value" : true, "version" : "6.0" }

An example of it is here and here.

[AlekSi]

Created FerretDB/FerretDB#2822 to track that