Use WORKSPACE variable to determine $__CODE_BASE in fallback

It was hard to run the tests locally on a non-wmf machine, as
$__CODE_BASE defaulted to "/a/...". Hence, we now generalize the
Jenkins case and build $__CODE_BASE from the WORKSPACE environment
variable.  Thereby, it should be possible to rune the tests an
non-gallium Jenkins slaves.


Change-Id: I2ba2c3e4ba829dd36017aa6ab9c303536e445662