use correct service-worker scope; add tests #1427

All 125 tests pass in 15s

125 tests +7 125 ✔️ +7 15s ⏱️ -2s
  15 suites +1     0 💤 ±0
    1 files ±0     0 ❌ ±0

Results for commit 6aac976. ± Comparison against earlier commit a1680fc.