CI: retry testem ci once on failure

The Browserstack job occasionally fails on transient browser-side
network errors (e.g. Safari `TypeError: Load failed` seen in PR #32
run 24655116088). Those failures come from the BrowserStack tunnel,
not from code under test.

On first failure, re-run the whole suite once; if that also fails
the original exit code propagates and the job fails normally.

Tradeoff: masks flaky tests with <=50% failure rate. Acceptable
here because the observed browserstack-only flakes are infra, not
product. For product flakiness we'd want per-test retry, which
testem does not provide natively.

Rejected alternatives:
- `retry_count: 1` in testem config — not a real testem option
  (verified against testem 3.19.1 source and docs); silently
  ignored.
- `.catch()` on `ember browserstack:results` / `:disconnect` —
  `process.exit(0)` on the success path skips the finally entirely,
  so the cleanup `.catch()` only fires when testem already failed.
  The linked PR #32 failure was a real test error, not a cleanup
  failure; the `.catch()` would not have changed the outcome.

Author: johanrd

bin/run-browserstack-tests.js

@@ -17,15 +17,26 @@ async function run(command, args = []) {
       // Calling testem directly here instead of `ember test` so that
       // we do not have to do a double build (by the time this is run
       // we have already ran `ember build`).
-      await run('testem', [
+      const testemArgs = [
         'ci',
         '-f',
         'testem.browserstack.js',
         '--host',
         '127.0.0.1',
         '--port',
         '7774',
-      ]);
+      ];
+
+      // One whole-suite retry to absorb transient BrowserStack network
+      // failures (e.g. Safari `TypeError: Load failed`). Masks flaky
+      // tests with <=50% failure rate; acceptable for this browserstack
+      // job because observed flakes are infra, not product.
+      try {
+        await run('testem', testemArgs);
+      } catch (e) {
+        console.log(chalk.yellow('testem ci failed; retrying once.'));
+        await run('testem', testemArgs);
+      }
 
       console.log('success');
       process.exit(0); // eslint-disable-line n/no-process-exit