test.sh

This introduces the first sample profiler driver (Stackdriver),
allowing initializing any Go service that uses labkit (and
upgrades to the new version) with the GITLAB_CONTINUOUS_PROFILING
env variable as such:

```
GITLAB_CONTINUOUS_PROFILING="stackdriver?service=gitaly&service_version=1.0.1&project_id=test-123" go run file.go
```

Passing GITLAB_CONTINUOUS_PROFILING=stackdriver (without further params)
will make the Stackdriver profiler guess these values based certain
environment criterias described at https://godoc.org/cloud.google.com/go/profiler#Config.

In order to use the profiler, users of labkit will need to compile it with the
`-tags continuous_profiler_stackdriver`, otherwise no dependencies will be
added and GITLAB_CONTINUOUS_PROFILING will take no effect.