Add custom request to return a list of all the tests in a project with locations

[adam-fowler]

This is something that would be used by the Visual Studio Code extension to provide location information for tests in the Test Explorer.

Currently the extension uses the swift test --list-tests command to get a list of tests, but this doesn't return locations of the tests. I did add a request swiftlang/swift-package-manager#5601 to add this information to the --list-tests output. But it seems it has been decided it would be easier to do this using SourceKit-LSP.

[ahoppen]

rdar://98710526