Fix name/path separator for telemetry_unittests test names (src-side)

This ports https://codereview.chromium.org/674023002 to src-side script.

BUG=399371, 425694, 422235
R=sergiyb@chromium.org

Review URL: https://codereview.chromium.org/677393002

Cr-Commit-Position: refs/heads/master@{#301351}
2 files changed