ongoing clean-up work (see https://codereview.webrtc.org/2887093002, https://codereview.webrtc.org/2894583002/ and https://codereview.webrtc.org/2891923002/), ReadDirectory() has been added in webrtc/test/testsupport/fileutils.h. BUG=NOBUG Review-Url: https://codereview.webrtc.org/2898753002 Cr-Commit-Position: refs/heads/master@{#18369}