Move the test template to //testing/test.gni (part 2)

sandbox/linux/BUILD.gn

Index: sandbox/linux/BUILD.gn
diff --git a/sandbox/linux/BUILD.gn b/sandbox/linux/BUILD.gn
index af3a95063997a2d9962978df5a17b444725c1eb0..a96f57ab5cecea98e257f089ac72e010e45598e0 100644
--- a/sandbox/linux/BUILD.gn
+++ b/sandbox/linux/BUILD.gn
@@ -3,6 +3,7 @@
 # found in the LICENSE file.
 
 import("//build/config/features.gni")
+import("//testing/rules.gni")
 
 declare_args() {
   compile_suid_client = is_linux
@@ -63,7 +64,8 @@ source_set("sandbox_linux_test_utils") {
 }
 
 # The main sandboxing test target.
-test("sandbox_linux_unittests") {
+executable("sandbox_linux_unittests") {
+  testonly = true
   sources = [
     "services/scoped_process_unittest.cc",
     "services/syscall_wrappers_unittest.cc",