Multiple test backends

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By default, test commands go to all backends (in sort order). To communicate with just a single backend, the first argument to the command must be -Xbackend (no space) where backend is the name in the configuration file (to be documented by the project). For example, a project with this configuration:

required_access_level: "maintainer"
backends:
  buildbot:
    backend: jobs
    config: …
  gitlab-ci:
    backend: pipelines

would have Do: test -Xbuildbot … and Do: test -Xgitlab-ci … communicate with the respective backends only (commands with unrecognized backends end up being ignored). Do: test and Do: test --merged would communicate with both in sort order of their backend names. In the latter case, the buildbot backend would first do the merge and drop the job and then the gitlab-ci backend would error out not knowing what the --merged flag is. Infrastructure for postponing the actual command until ready is not supported (though may be possible with a wrapper around HostingService that passed read-only commands through, but buffered write actions until ready). The jobs backend would still be queue the job since that occurs outside the service trait's purview.