Add flag --require-audio-hardware-for-testing.

When enabled, tests that require audio hardware will fail if the system
doesn't provide it. Otherwise, they will silently succeed.

TEST=media_unittests (and AudioInputTests with !InputDevicesAvailable())
BUG=432624

Review URL: https://codereview.chromium.org/914483002

Cr-Commit-Position: refs/heads/master@{#315823}