Native test needed for unhandled symlink scenario

Opening this issue to track an internal work item, to add a test for the scenario of opening the same file via multiple symlinks, on Linux and Mac.

The underlying issue is that we resolve symlinks when canonicalizing paths on Linux and Mac (a result of calling realpath), so wind up considering those paths to refer to the same file, while VS Code does not resolve them to the same file. This causes some issues with various features that need to match file paths back to VS Code URI's.

We have/had a test (invalid_open_file_instance_winonly) which exercised the same error handling, but that test was based on varying URI casing and not symlinks. It's no longer a valid scenario as VS Code has since ensured their URI casings on Windows are consistent/canonical.

Locate the existing invalid_open_file_instance_winonly test and its surrounding native tests. Use that test as the starting point for covering the same error handling with one file opened through multiple symlinks on Linux and macOS; done means the scenario is exercised on both platforms.