marionette-harness is implemented as a "generic" task and
can be triggered with the following try syntax:
`-p linux64_tc -j marionette-harness -u none -t none`
The Marionette harness tests do not depend on a browser build,
they only exercise Python code.
Add option to testing/docker/desktop-build/bin/build.sh to
execute any in-tree script instead of only `build-linux.sh`.
This feature is used to call
testing/taskcluster/scripts/tester/harness-test-linux.sh, which
in turn executes the mozharness script specified in the environment.
MozReview-Commit-ID: H08v8ow4p8q
212e0b801a