Use logdog butler subcommand to run tests.

Previously we ran tests, without setting butler environment variables.
This will run into NotBootstrappedError when we try to upload test
results through logdog.

In this cl, we use logdog butler subcommand to run tests, which will set
butler environment variables before hand.

BUG=692287

Review-Url: https://codereview.chromium.org/2695963003
Cr-Commit-Position: refs/heads/master@{#456976}
9 files changed