Fix flaky macOS CI from over-long test IPC socket path

`@vscode/test-cli` defaults `--user-data-dir` to `.vscode-test/user-data`
inside the workspace. On CI the workspace is nested three deep
(`.../vscode-powershell/vscode-powershell/vscode-powershell/`), so the VS
Code instance's IPC socket (`<user-data-dir>/<version>-main.sock`) exceeds
macOS's 104-char `AF_UNIX` limit and the editor dies at startup with
`listen EINVAL`, flaking the whole matrix (and cancelling the other OSes
via fail-fast).

Anchor `--user-data-dir` in `os.tmpdir()` so the socket path stays short
on every platform.

Drafted by Copilot (Claude Opus 4.8).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: andyleejordan

.vscode-test.mjs

@@ -1,5 +1,7 @@
 import { defineConfig } from "@vscode/test-cli";
 import { existsSync } from "fs";
+import { tmpdir } from "os";
+import { join } from "path";
 
 export default defineConfig({
     files: "test/**/*.test.ts",
@@ -10,6 +12,12 @@ export default defineConfig({
         "--disable-extensions",
         // Undocumented but valid option to use a temporary profile for testing
         "--profile-temp",
+        // The default user-data-dir lives under the (deeply nested on CI)
+        // workspace path, and its IPC socket blows past macOS's 104-char
+        // unix-socket limit, causing a flaky `listen EINVAL`. Anchor it in the
+        // OS temp dir so the socket path stays short on every platform.
+        "--user-data-dir",
+        join(tmpdir(), "vscode-powershell-test"),
     ],
     workspaceFolder: `test/${existsSync("C:\\powershell-7\\pwsh.exe") ? "OneBranch" : "TestEnvironment"}.code-workspace`,
     mocha: {