Add target_sdk_version to rtc_test.
Add target_sdk_version to rtc_test.

Starting fom crrev.com/c/2290319, the build system will start to
enforce the presence of target_sdk_version. This CL sets it to 23 to
keep the current behaviour.

TBR=titovartem@webrtc.org

Bug: None
Change-Id: If9b9d730c3899c4e6afbd6f08a56b5a73cf126a1
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/179084
Reviewed-by: Mirko Bonadei <d2c43c210eae6feef04f53bae50885e8152edcca@webrtc.org>
Commit-Queue: Mirko Bonadei <d2c43c210eae6feef04f53bae50885e8152edcca@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#31705}

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