testing: work on stack trace and adoption in selfhost test provider

Builds on #221225, only the last commit needs review.

Still a WIP, this sketches things out for proper implementation tomorrow.

.vscode/extensions/vscode-selfhost-test-provider/tsconfig.json

@@ -11,6 +11,7 @@
 		"src/**/*",
 		"../../../src/vscode-dts/vscode.d.ts",
 		"../../../src/vscode-dts/vscode.proposed.testObserver.d.ts",
+		"../../../src/vscode-dts/vscode.proposed.testMessageStackTrace.d.ts",
 		"../../../src/vscode-dts/vscode.proposed.attributableCoverage.d.ts"
 	]
 }