Enable screen capturer tests for Linux / DirectX capturer / magnifier capturer

GDI capturer may randomly return a blank frame. So this change enables tests for
Linux / DirectX capturer / magnifier capturer.

BUG=webrtc:6666

Review-Url: https://codereview.webrtc.org/2559583002
Cr-Original-Commit-Position: refs/heads/master@{#15489}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: e83f4b383551b9ac1c1549e48cbd7a1664ce8cde
1 file changed