Introduce new properties for correct discovery completion

[Sanan07]

Description

1. Re-introduce properties for FullyDiscoveredSources, PartiallyDiscoveredSources and NotDiscoveredSources in DiscoveryCompleteEventArgs.
2. Add logic to test host to handle parallel discovery. Since parallel discovery happens at a source level and since each test host is only discovering one source at a time, we should be able to do this by tracking the TestCases being returned from adapter and marking a source as ‘FullyDiscovered’ when we see that the source for a TestCase is not the same as that for the previous TestCase. All sources can start off as ‘NotDiscovered’ and a source can be marked ‘PartiallyDiscovered’ when we receive first TestCase for this source from adapter.
3. This will require to bump up the protocol version, since old test hosts (referenced from .NET Core test projects that reference old version of Test SDK package) will not support the logic in 2.
4. If test host does not support the above (i.e. if protocol version is older):
   o We will run discovery sequentially even if the supplied .runsettings from user’s solution specifies a MaxCpuCount node. This means, test console will need a change to ignore / remove the MaxCpuCount node.
   o In this case, test console will also need similar change as 2 above to decide when a source is `FullyDiscovered’. Only difference may be that it would need to track the discovery completion by looking inside the sequential TestCase chunks it receives from the test host.

Corner cases:
• A source can technically contain tests that can be discovered by multiple adapters (e.g. both xUnit and MSTest tests in the same binary). It would be great to support this if this is feasible and easy – but we could say that this is an unsupported scenario given we haven’t heard much about such cases in feedback.

Partially implemented #

[shyamnamboodiripad]

Note: This work is a pre-requisite for correctly supporting parallel discovery.

[shyamnamboodiripad]

fyi @Haplois

[Evangelink]

Closed by #3349