Switch to class-scoped collection names in the tests

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All tests currently use the same Qdrant collection name, and the initialization code (which runs before each test) removes all collections from Qdrant. This makes it impossible to run test classes in parallel (the default xunit behavior).

We can have class-scoped collections to allow for more parallelism.

Discussed in https://github.com/qdrant/qdrant-dotnet/pull/11#discussion_r1373175534