Use RTC_DISALLOW_COPY_AND_ASSIGN in webrtc/base/sigslottester.h

It was incorrectly using a older version of the macro, which
wasn't discovered since the code wasn't built in WebRTC until now.

I moved webrtc/base/sigslottester.h from rtc_unittests into
rtc_base_test_utils instead to make it more usable.

BUG=webrtc:6821

Review-Url: https://codereview.webrtc.org/2551813002
Cr-Original-Commit-Position: refs/heads/master@{#15422}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 1d08100b9e1db449c943ce64663c10a019834a09
4 files changed