Output test results in xunit format when the path is 'xunit'

We output via console.log; to differentiate from normal output,
we prefix xunit output with 'XUNIT '.
Get S3 credentials from env variables, if set, in publish-release.js
Allow TIMEOUT_SCALE_FACTOR env variable to scale up timeouts for self-test

Particularly for automated tests, where we may run on a slow machine, we need
an escape valve to let us boost the timeouts.  This also allows for a 'tolerant'
test (scale factor 2+?) and a 'strict' test (scale factor 0.5?) etc
By default, don't run server & client tests in parallel