The new approach has a few advantages over the olde TIBS-based approach:
1. The source file being tested is defined within the test case itself and not in a separate file, which makes it easier to understand the test case since the auxiliaury file doesn’t need to be opened. Finding it inside `Sources/SKTestSupport/INPUTS` is already hard for people that are not familiar with the codebase.
2. The build setup is significantly simpler since it doesn’t rely on `ninja`. It is thus easier to understand what is run during the test.
3. We can use the emoji location markers to refer to test locations, like we do for files that are opened using `TestSourceKitLSPClient.openDocument`.
This commit only migrates call hierarchy testing to the new design. If we like it, I’ll migrate the other test workspaces as well.
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