feat(test): add --retry flag and emit separate testcase entries for retries in JUnit XML (#26866)

#### (Copies commits from #26447)

## Summary
- Add a global `--retry <N>` flag to `bun test` that sets a default
retry count for all tests (overridable by per-test `{ retry: N }`). Also
configurable via `[test] retry = N` in bunfig.toml.
- When a test passes after one or more retries, the JUnit XML reporter
emits a separate `<testcase>` entry for each failed attempt (with
`<failure>`), followed by the final passing `<testcase>`. This gives
flaky test detection tools per-attempt timing and result data using
standard JUnit XML that all CI systems can parse.

## Test plan
- `bun bd test test/js/junit-reporter/junit.test.js` — verifies separate
`<testcase>` entries appear in JUnit XML for tests that pass after retry
- `bun bd test test/cli/test/retry-flag.test.ts` — verifies the
`--retry` CLI flag applies a default retry count to all tests

## Changelog
<!-- CHANGELOG:START -->
- Added `--retry <N>` flag to `bun test` to set a default retry count
for all tests
- Added `[test] retry` option to bunfig.toml
- JUnit XML reporter now emits separate `<testcase>` entries for each
retry attempt, providing CI visibility into flaky tests
<!-- CHANGELOG:END -->

---------

Co-authored-by: Chris Lloyd <chrislloyd@anthropic.com>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

src/cli/test_command.zig

@@ -854,6 +854,9 @@ pub const CommandLineReporter = struct {
                         }
                     }
 
+                    for (sequence.flakyAttempts()) |attempt| {
+                        bun.handleOom(junit.writeTestCase(attempt.result, filename, display_label, concatenated_describe_scopes.items, 0, attempt.elapsed_ns, line_number));
+                    }
                     bun.handleOom(junit.writeTestCase(status, filename, display_label, concatenated_describe_scopes.items, assertions, elapsed_ns, line_number));
                 }
             }