Revert of Move gn _run target generation into test(). (patchset #1 id:1 of https://codereview.chromium.org/1653003006/ )

Reason for revert:
Broke angle_unittests_run, which doesn't follow the template:

https://build.chromium.org/p/chromium.gpu.fyi/builders/GPU%20Linux%20Builder/builds/53646/steps/compile/logs/stdio

Original issue's description:
> Move gn _run target generation into test().
>
> TBR=brettw@chromium.org
> BUG=
> CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel
>
> Committed: https://crrev.com/6b1b03783b81ea27bb521c02aa84b74f06d6f71a
> Cr-Commit-Position: refs/heads/master@{#373232}

TBR=phosek@chromium.org,brettw@chromium.org,jbudorick@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=

Review URL: https://codereview.chromium.org/1664653003

Cr-Original-Commit-Position: refs/heads/master@{#373257}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 002f2226477126a477d38086b119073911fa231d
diff --git a/tools/gn/BUILD.gn b/tools/gn/BUILD.gn
index eb60e08..e643f97 100644
--- a/tools/gn/BUILD.gn
+++ b/tools/gn/BUILD.gn
@@ -241,6 +241,15 @@
   ]
 }
 
+# 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",