Test count metrics

[finestructure]

We added test target metrics a while ago and discussed ideas around further test metrics.

One of these was a test count.

Concerning this, Paul LeMarquand from the Swift Testing working group suggested to use lsp-server to index a project's tests and then query it for the number of tests it found. This would work for both XCTest and Swift Testing tests/targets and should give exactly the same results as seen in Xcode or VS Code.

The LSP server would be sent a workspace/_synchronize request to start indexing, followed by a workspace/tests request for the test hierarchy. (Full request spec)

We’d also want to disable background indexing in sourcekit-lsp, which would let the workspace/_pollIndex command kick off indexing once and then stop watching the workspace when it finishes.

One option to interface with LSP would be a client package like LanguageClient.