Package logstream lets end-to-end tests incorporate controller logs
into the error output of tests. It is enabled by setting the
SYSTEM_NAMESPACE environment variable, which tells this package
what namespace to stream logs from.
Start begins streaming the logs from system components with a `key:` matching
`test.ObjectNameForTest(t)` to `t.Log`. It returns a Canceler, which must
be called before the test completes.