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

This is a reland of https://codereview.chromium.org/1653003006/

Consolidating the _run targets into the test template should make them
easier to maintain (and, eventually, remove).

BUG=583694
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1662053002

Cr-Original-Commit-Position: refs/heads/master@{#373385}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 3c0ef4309a77bbf172f081c99bbb33e732ea07af

tools/gn/BUILD.gn

diff --git a/tools/gn/BUILD.gn b/tools/gn/BUILD.gn
index e643f97..eb60e08 100644
--- a/tools/gn/BUILD.gn
+++ b/tools/gn/BUILD.gn
@@ -241,15 +241,6 @@
   ]
 }
 
-# TODO(GYP): Delete this after we've converted everything to GN.
-# The _run targets exist only for compatibility w/ GYP.
-group("gn_unittests_run") {
-  testonly = true
-  deps = [
-    ":gn_unittests",
-  ]
-}
-
 test("gn_unittests") {
   sources = [
     "action_target_generator_unittest.cc",