[Video Capture, Test] Make tests requiring real webcam fail if it's not found

Use prefix "UsingRealWebcam_" for names of tests that require real webcam and
add checking whether webcam is found or not.
Accordingly, exclude such test cases from test suites for devices that have no
camera.

Bug: 893494
Test: Ran capture_unittests including UsingRealWebcam_ tests on Chromebox (guado)
      and it failed.
Test: Ran capture_unittests excluding UsingRealWebcam_ tests on Chromebox (guado)
      and it passed.
Test: Ran capture_unittests including UsingRealWebcam_ tests on Chromebook (kevin)
      and it passed.

Change-Id: I5c91380b7674aeace6a7d1764f7fad31f02f8af7
Reviewed-on: https://chromium-review.googlesource.com/c/1276407
Commit-Queue: Keiichi Watanabe <keiichiw@chromium.org>
Reviewed-by: Christian Fremerey <chfremer@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599097}
11 files changed