Move gn _run target generation into test(). (RELAND)

This is a reland of https://codereview.chromium.org/1653003006/

Consolidating the _run targets into the test template should make them
easier to maintain (and, eventually, remove).

BUG=583694
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1662053002

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