[yarn]: Add yarn_unittests GN target to BUILD.gn
[yarn]: Add yarn_unittests GN target to BUILD.gn

Change-Id: I8f9324f40343d84c57bca76a33d1ce98311b8bf4
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/35931
Kokoro-Presubmit: David Turner <dcb93bd72059500c1226ad16fdd58ffe25c9e0ea@google.com>
Tested-by: David Turner <dcb93bd72059500c1226ad16fdd58ffe25c9e0ea@google.com>
Presubmit-Ready: David Turner <dcb93bd72059500c1226ad16fdd58ffe25c9e0ea@google.com>
Reviewed-by: Ben Clayton <1f693eb7158dc33015048c09d6712f74dabddf79@google.com>

bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,google/swiftshader,google/swiftshader,bkaradzic/SwiftShader