Move gn _run target generation into test(). (RELAND)

testing/test.gni

Index: testing/test.gni
diff --git a/testing/test.gni b/testing/test.gni
index acd1e1acab3408cca6403648d5efa88f4212caeb..1c2a927717f99bd565aaa2db1ed20d0919b2f263 100644
--- a/testing/test.gni
+++ b/testing/test.gni
@@ -186,6 +186,8 @@ template("test") {
       ]
     }
   } else {
+    main_target_name = target_name
+
     executable(target_name) {
       forward_variables_from(invoker, "*")
 
@@ -203,5 +205,14 @@ template("test") {
         "//build/win:default_exe_manifest",
       ]
     }
+
+    # TODO(GYP): Delete this after we've converted everything to GN.
+    # The _run targets exist only for compatibility with GYP.
+    group("${target_name}_run") {
+      testonly = true
+      deps = [
+        ":$main_target_name",
+      ]
+    }
   }
 }