This surfaces test failures more clearly on CircleCI by adding
testem-multi-reporter to report both via the default TAP reporter
as well as an xUnit reporter whose output is stored as an artefact.
This adopts pattern used by Vault, where we split CircleCI yaml config
into multiple files that get packed and translated to 2.0.
This has two motivations: First, to ease translating config to CircleCI
2.0 so it can run on Enterprise private repository. Second and most
importantly, it also adding Enterprise specific jobs in separate files
with reduced config file merging conflict resolution.
