fix(test): resolve the macOS temp root through its symlink in the launcher and preview tests (#501)

Supersedes #301 by @VassagoDevteam, ported onto the current component layout with fixes from review.

Verified before merge: format, package boundaries, unit suite, build, and the browser tracking suite; a browser harness for the change where one applies; adversarial review applied.

Co-authored-by: Vassago DevTeam <[email protected]>

src/tooling/fixtureRoot.mjs

@@ -0,0 +1,21 @@
+import { mkdtemp, realpath } from 'node:fs/promises';
+import { tmpdir } from 'node:os';
+import path from 'node:path';
+
+/**
+ * Create a temporary directory for a test fixture, resolved to its physical
+ * path.
+ *
+ * The system temp directory is reached through a symlink on macOS
+ * (`/var` -> `/private/var`, `/tmp` -> `/private/tmp`). A test that gives the
+ * logical path to something that reports paths back — a launched process's
+ * working directory, a bundler's project root — then compares two spellings of
+ * the same directory and fails. Resolving the root once, here, keeps both sides
+ * on the physical path.
+ *
+ * @param {string} prefix - Directory name prefix, e.g. `gev-preview-`.
+ * @returns {Promise<string>} The fixture root, with no symlinked component.
+ */
+export async function makeFixtureRoot(prefix) {
+  return realpath(await mkdtemp(path.join(tmpdir(), prefix)));
+}