Use headless mode for the "Run current file integration test" vscode task

For running an integration test just to see if it fails or succeeds,
headless mode is sufficient and actually better, because it works in
small terminals like vscode's bottom panel; the "main.go cli" way of
running tests tends to fail there because the layout renders differently
in such a small window. Headless tests use a fixed window size, so they
don't have this problem. It's also slightly faster.

The other vscode tasks (slow and sandbox) are unchanged, they only make
sense with a visible UI.

.vscode/tasks.json

@@ -24,7 +24,7 @@
     {
       "label": "Run current file integration test",
       "type": "shell",
-      "command": "go run cmd/integration_test/main.go cli ${relativeFile}",
+      "command": "just e2e ${relativeFile}",
       "problemMatcher": [],
       "group": {
         "kind": "test",