Investigate loading behaviour for Test Explorer

[alexcrocha]

We observed that the Test Explorer behaves differently whether the indexer has completed or not.
We may have to ensure the Test Explorer items populate only after the indexer completes.

class ActiveSupportTimeExtSpec < ::DslSpec was not populating correctly when we opened the Test Explorer panel too quickly after a VS Code reload, because the ancestor could not be linearized in time.

[andyw8]

(the above only applies when the fullTestDiscovery flag is enabled)

[andyw8]

Udpate: This may already be fixed by #3203, we will verify.