Create sandbox_unittests target

This target includes //sandbox/policy:tests, a source_set, on all
platforms. The current dependency in Linux, Mac, and Windows will
be pointed to this newly created unit test target in the following CLs.

Bug: 1297869
Change-Id: Ife20aa74b295badc728dc4ae26faeb7a7ba7ba93
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3719086
Commit-Queue: Guocheng Wei <[email protected]>
Reviewed-by: Robert Sesek <[email protected]>
Reviewed-by: Sylvain Defresne <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1018786}
diff --git a/BUILD.gn b/BUILD.gn
index 9c5c055..cea04c3 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -87,6 +87,7 @@
     "//components/gwp_asan:gwp_asan_unittests",
     "//infra/orchestrator:orchestrator_all",
     "//net:net_unittests",
+    "//sandbox:sandbox_unittests",
     "//services:services_unittests",
     "//services/service_manager/public/cpp",
     "//skia:skia_unittests",