[Chromecast] Run net_unittests in batches of 25.
[Chromecast] Run net_unittests in batches of 25.

There is high overhead to running one batch of net_unittests (on
the order of about 1s per batch). To reduce the amount of time it
takes to run the test suite, raise the number of test run per batch
from 10 to 25.

BUG= internal b/23156294

Review URL: https://codereview.chromium.org/1320973003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#346019}

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