use (num_cores/2) jobs by default for telemetry unittests.
use (num_cores/2) jobs by default for telemetry unittests.

Previously we were using (num_cores/4). Let's see if we can get more
parallelism and still not overload the machine.

R=dtu@chromium.org
BUG=388256

Review URL: https://codereview.chromium.org/742373002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#305163}

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