js-on-sx: bump test262 runner per-test timeout 5s→15s

With 4 parallel workers contending, the 5s default timed out 85/99
built-ins/String tests. Bumping to 15s yields 65/99 (65.7%) with
real failure modes now visible instead of "85x Timeout".

lib/js/test262-scoreboard.md

@@ -1,31 +1,31 @@
 # test262 scoreboard
 
 Pinned commit: `d5e73fc8d2c663554fb72e2380a8c2bc1a318a33`
-Wall time: 157.9s
+Wall time: 420.4s
 
-**Total:** 66/100 runnable passed (66.0%). Raw: pass=66 fail=25 skip=1130 timeout=9 total=1230.
+**Total:** 65/99 runnable passed (65.7%). Raw: pass=65 fail=26 skip=1 timeout=8 total=100.
 
 ## Top failure modes
 
-- **14x** Test262Error (assertion failed)
-- **9x** Timeout
+- **16x** Test262Error (assertion failed)
+- **8x** Timeout
 - **6x** TypeError: not a function
-- **2x** ReferenceError (undefined symbol)
 - **2x** Unhandled: Not callable: \\\
+- **1x** ReferenceError (undefined symbol)
 - **1x** SyntaxError (parse/unsupported syntax)
 
 ## Categories (worst pass-rate first, min 10 runnable)
 
 | Category | Pass | Fail | Skip | Timeout | Total | Pass % |
 |---|---:|---:|---:|---:|---:|---:|
-| built-ins/String | 66 | 25 | 1123 | 9 | 1223 | 66.0% |
+| built-ins/String | 65 | 26 | 1 | 8 | 100 | 65.7% |
 
 ## Per-category top failures (min 10 runnable, worst first)
 
-### built-ins/String  (66/100 — 66.0%)
+### built-ins/String  (65/99 — 65.7%)
 
-- **14x** Test262Error (assertion failed)
-- **9x** Timeout
+- **16x** Test262Error (assertion failed)
+- **8x** Timeout
 - **6x** TypeError: not a function
-- **2x** ReferenceError (undefined symbol)
 - **2x** Unhandled: Not callable: \\\
+- **1x** ReferenceError (undefined symbol)