Restore previously loaded test cases

[mnoah1]

This change will persist the prior list of test cases as part of the workspace state, and restore this data when the test explorer relaunches. This avoids a wait for the full BSP build target request before test cases become visible:

• Update the test case store to cache the build targets result and each target sources result. When a refresh runs, the latest version of these get stored.
• Before registering the handlers, there is now an additional step to pull these stored test cases and re-add them back to the test explorer.
• These get processed through the usual test case creation logic, so that a test explorer tree created from a restored result or a new one from the build server will behave the same way.
• The refresh run always clears out any cached values, so these are only used when restoring the window, and an actual refresh run is always guaranteed to contain the latest results from the build server.