Specify inputs and testonly in Go GN rules
Specify inputs and testonly in Go GN rules

This marks the go_test_binary targets as testonly so they can depend on
other testonly things, such as the test mojom targets, and marks the
sources to the go rules as 'inputs' to the GN action so that ninja knows
to rerun the go step when a go file specified as a source changes. This
does not handle changes to things that the go tool finds to import (for
that we'll need a script that generates a .d file) but simplifies the
common workflow so is probably worth having in the meantime.

R=rogulenko@google.com

Review URL: https://codereview.chromium.org/979693002

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