Use /b in testing CWD
Use /b in testing CWD

Needed for https://codereview.chromium.org/2102503002

BUG=chromium:584079, chromium:459840, chromium:593999, chromium:596887

Review-Url: https://codereview.chromium.org/2095353002

luci/recipes-py,luci/recipes-py