Adds flag to customize output root dir in scenario tests.
Adds flag to customize output root dir in scenario tests.

Bug: webrtc:9510
Change-Id: I005baffc0b0166dc39f5f387b1d3b5caa1f350a9
Reviewed-on: https://webrtc-review.googlesource.com/c/115200
Reviewed-by: Christoffer Rodbro <a8869a0b8551100ac7ab874b7705be1f60ffd43b@webrtc.org>
Commit-Queue: Sebastian Jansson <c24d1bc144eb7c11e553f59153fa17dba85ca83e@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#26071}
ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc