Add ability to instantiate VideoEncoderSoftwareFallback in VP tests. * Split TestConfig::hw_codec into hw_encoder and hw_decoder. * Add TestConfig::sw_fallback_encoder. BUG=webrtc:6634 Review-Url: https://codereview.webrtc.org/3009963002 Cr-Original-Commit-Position: refs/heads/master@{#19710} Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc Cr-Mirrored-Commit: d635e5b8ed2e77d4dac6b08d0f33f6fba466632e